Restore the community photo after reload and reject empty, oversized, SVG, and spoofed uploads.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/(app)/create/components/CreateFlowPendingAvatarFlush.tsx

@@ -2,7 +2,10 @@
 
 import { useEffect, useRef } from "react";
 import { useCreateFlow } from "../context/CreateFlowContext";
-import { uploadCreateFlowFile } from "../../../../lib/create/uploadToServer";
+import {
+  CreateFlowUploadValidationError,
+  uploadCreateFlowFile,
+} from "../../../../lib/create/uploadToServer";
 import {
   clearPendingCommunityAvatarFile,
   readPendingCommunityAvatarFile,
@@ -19,13 +22,17 @@ export function CreateFlowPendingAvatarFlush({
   sessionUser: { id: string; email: string } | null | undefined;
   sessionResolved: boolean;
 }) {
-  const { updateState } = useCreateFlow();
+  const { state, updateState } = useCreateFlow();
   /** One successful flush per signed-in user id (survives React StrictMode remounts). */
   const lastFlushedUserIdRef = useRef<string | null>(null);
+  const hasServerAvatar =
+    typeof state.communityAvatarUrl === "string" &&
+    state.communityAvatarUrl.trim().length > 0;
 
   useEffect(() => {
     if (!sessionResolved || !sessionUser) return;
     if (lastFlushedUserIdRef.current === sessionUser.id) return;
+    if (hasServerAvatar) return;
     let cancelled = false;
 
     void (async () => {
@@ -37,15 +44,18 @@ export function CreateFlowPendingAvatarFlush({
         await clearPendingCommunityAvatarFile();
         updateState({ communityAvatarUrl: url });
         lastFlushedUserIdRef.current = sessionUser.id;
-      } catch {
+      } catch (err) {
-        // Leave pending blob in place so the user can retry after fixing auth / UPLOAD_ROOT.
+        if (err instanceof CreateFlowUploadValidationError) {
+          await clearPendingCommunityAvatarFile();
+        }
+        // Leave a transient (auth / UPLOAD_ROOT) failure in place to retry.
       }
     })();
 
     return () => {
       cancelled = true;
     };
-  }, [sessionResolved, sessionUser, updateState]);
+  }, [hasServerAvatar, sessionResolved, sessionUser, updateState]);
 
   return null;
 }

app/(app)/create/components/CustomMethodCardFieldBlocksSummary/CustomMethodCardUploadBlockRow.container.tsx

@@ -3,7 +3,10 @@
 import { memo, useCallback, useRef, useState } from "react";
 import { useTranslation } from "../../../../contexts/MessagesContext";
 import type { CustomMethodCardFieldBlock } from "../../../../../lib/create/customMethodCardFieldBlocks";
-import { uploadCreateFlowFile } from "../../../../../lib/create/uploadToServer";
+import {
+  createFlowUploadFailureMessageKey,
+  uploadCreateFlowFile,
+} from "../../../../../lib/create/uploadToServer";
 import { CustomMethodCardUploadBlockRowView } from "./CustomMethodCardUploadBlockRow.view";
 import type { CustomMethodCardUploadBlockRowProps } from "./CustomMethodCardFieldBlocksSummary.types";
 
@@ -68,8 +71,8 @@ function CustomMethodCardUploadBlockRowContainerComponent({
                 : b,
             ),
           );
-        } catch {
+        } catch (err) {
-          setErrorMessage(tUpload("errors.generic"));
+          setErrorMessage(tUpload(createFlowUploadFailureMessageKey(err)));
         } finally {
           setBusy(false);
         }

app/(app)/create/components/CustomMethodCardFieldBlocksSummary/CustomMethodCardUploadBlockRow.view.tsx

@@ -4,6 +4,7 @@ import { memo } from "react";
 import Upload from "../../../../components/controls/Upload";
 import InputLabel from "../../../../components/type/InputLabel";
 import { ASSETS, getAssetPath } from "../../../../../lib/assetUtils";
+import { CUSTOM_ATTACHMENT_ACCEPT } from "../../../../../lib/create/createFlowUploadValidation";
 import type { CustomMethodCardUploadBlockRowViewProps } from "./CustomMethodCardFieldBlocksSummary.types";
 
 function CustomMethodCardUploadBlockRowViewComponent({
@@ -43,7 +44,7 @@ function CustomMethodCardUploadBlockRowViewComponent({
         type="file"
         className="sr-only"
         tabIndex={-1}
-        accept="image/jpeg,image/png,image/webp,image/gif,application/pdf"
+        accept={CUSTOM_ATTACHMENT_ACCEPT}
         aria-label={uploadFileInputAriaLabel}
         onChange={onFileInputChange}
       />

app/(app)/create/components/CustomMethodCardWizard/CustomMethodCardWizard.container.tsx

@@ -8,6 +8,7 @@ import {
 import { useAsyncConfirm } from "../../../../hooks/useAsyncConfirm";
 import { useBeforeUnloadGuard } from "../../../../hooks/useBeforeUnloadGuard";
 import type { CustomMethodCardFieldBlock } from "../../../../../lib/create/customMethodCardFieldBlocks";
+import { createFlowUploadFailureMessageKey } from "../../../../../lib/create/uploadToServer";
 import {
   CUSTOM_METHOD_CARD_WIZARD_MAX_DESCRIPTION_CHARS,
   CUSTOM_METHOD_CARD_WIZARD_MAX_FIELD_CHARS,
@@ -462,8 +463,9 @@ const CustomMethodCardWizardContainer = memo<CustomMethodCardWizardProps>(
         try {
           const { url } = await onPersistCustomUploadFile(file);
           setUploadAssetUrl(url);
-        } catch {
+        } catch (err) {
-          setUploadFieldError(tUpload("errors.generic"));
+          setUploadFileName(undefined);
+          setUploadFieldError(tUpload(createFlowUploadFailureMessageKey(err)));
         } finally {
           setUploadFieldBusy(false);
         }

app/(app)/create/components/CustomMethodCardWizard/CustomMethodCardWizardFieldBodies.view.tsx

@@ -10,6 +10,7 @@ import IncrementerBlock from "../../../../components/controls/IncrementerBlock";
 import InputLabel from "../../../../components/type/InputLabel";
 import ApplicableScopeField from "../ApplicableScopeField";
 import { CUSTOM_METHOD_CARD_WIZARD_MAX_FIELD_CHARS } from "../../../../../lib/create/customMethodCardWizardConstants";
+import { CUSTOM_ATTACHMENT_ACCEPT } from "../../../../../lib/create/createFlowUploadValidation";
 import type { CustomMethodCardWizardFieldBodiesViewProps } from "./CustomMethodCardWizard.types";
 
 const TEXT_PLACEHOLDER_MAX = 8000;
@@ -119,6 +120,7 @@ function CustomMethodCardWizardFieldBodiesViewComponent({
           type="file"
           className="sr-only"
           tabIndex={-1}
+          accept={CUSTOM_ATTACHMENT_ACCEPT}
           aria-label={copy.upload.uploadFileInputAriaLabel}
           onChange={onFileChosen}
         />

app/(app)/create/screens/upload/CommunityUploadScreen.tsx

@@ -16,14 +16,24 @@ import { CREATE_FLOW_MD_UP_COLUMN_MAX_CLASS } from "../../components/createFlowL
 import { fetchAuthSession } from "../../../../../lib/create/api";
 import { ASSETS, getAssetPath } from "../../../../../lib/assetUtils";
 import {
-  UploadToServerError,
+  createFlowUploadFailureMessageKey,
   uploadCreateFlowFile,
 } from "../../../../../lib/create/uploadToServer";
+import {
+  COMMUNITY_AVATAR_ACCEPT,
+  messageKeyForCreateFlowUploadReason,
+  validateCreateFlowUploadFile,
+} from "../../../../../lib/create/createFlowUploadValidation";
 import {
   clearPendingCommunityAvatarFile,
+  readPendingCommunityAvatarFile,
   storePendingCommunityAvatarFile,
 } from "../../../../../lib/create/pendingCommunityAvatarUpload";
 
+function hasCommunityAvatarUrl(url: string | undefined): boolean {
+  return typeof url === "string" && url.trim().length > 0;
+}
+
 /** Create Community — Figma Flow — Upload `20094:41524`. */
 export function CommunityUploadScreen() {
   const m = useMessages();
@@ -54,17 +64,57 @@ export function CommunityUploadScreen() {
     [localPreviewUrl],
   );
 
-  const resolveUploadError = useCallback(
+  const serverAvatarUrl = hasCommunityAvatarUrl(state.communityAvatarUrl)
-    (err: unknown) => {
+    ? state.communityAvatarUrl!.trim()
-      if (err instanceof UploadToServerError) {
+    : null;
-        if (err.status === 413) return tUpload("errors.tooLarge");
+  const serverAvatarUrlRef = useRef(serverAvatarUrl);
-        if (err.status === 401) return tUpload("errors.unauthorized");
+  serverAvatarUrlRef.current = serverAvatarUrl;
-        if (err.code === "server_misconfigured") {
+
-          return tUpload("errors.misconfigured");
+  useEffect(() => {
+    if (serverAvatarUrl) {
+      setLocalPreviewUrl((prev) => {
+        if (prev) URL.revokeObjectURL(prev);
+        return null;
+      });
+    }
+  }, [serverAvatarUrl]);
+
+  useEffect(() => {
+    let cancelled = false;
+    void (async () => {
+      try {
+        const file = await readPendingCommunityAvatarFile();
+        if (cancelled || !file) return;
+        const validated = await validateCreateFlowUploadFile(
+          file,
+          "communityAvatar",
+        );
+        if (validated.ok === false) {
+          await clearPendingCommunityAvatarFile();
+          return;
         }
+        if (cancelled) return;
+        if (serverAvatarUrlRef.current) return;
+        const objectUrl = URL.createObjectURL(file);
+        if (cancelled) {
+          URL.revokeObjectURL(objectUrl);
+          return;
+        }
+        setLocalPreviewUrl((prev) => {
+          if (prev) URL.revokeObjectURL(prev);
+          return objectUrl;
+        });
+      } catch {
+        // Missing IndexedDB / quota: leave the picker empty.
       }
-      return tUpload("errors.generic");
+    })();
-    },
+    return () => {
+      cancelled = true;
+    };
+  }, []);
+
+  const resolveUploadError = useCallback(
+    (err: unknown) => tUpload(createFlowUploadFailureMessageKey(err)),
     [tUpload],
   );
 
@@ -94,6 +144,16 @@ export function CommunityUploadScreen() {
       }
 
       if (signedIn === false) {
+        const validated = await validateCreateFlowUploadFile(
+          file,
+          "communityAvatar",
+        );
+        if (validated.ok === false) {
+          setErrorMessage(
+            tUpload(messageKeyForCreateFlowUploadReason(validated.reason)),
+          );
+          return;
+        }
         try {
           await storePendingCommunityAvatarFile(file);
           setLocalPreviewUrl((prev) => {
@@ -121,24 +181,16 @@ export function CommunityUploadScreen() {
       if (prev) URL.revokeObjectURL(prev);
       return null;
     });
-    if (
+    if (hasCommunityAvatarUrl(state.communityAvatarUrl)) {
-      typeof state.communityAvatarUrl === "string" &&
-      state.communityAvatarUrl.trim().length > 0
-    ) {
       updateState({ communityAvatarUrl: undefined });
     }
-    // Clear any anonymous staged blob so the post-sign-in flush won't resurrect it.
     void clearPendingCommunityAvatarFile();
     if (fileInputRef.current) {
       fileInputRef.current.value = "";
     }
   }, [markCreateFlowInteraction, state.communityAvatarUrl, updateState]);
 
-  const displaySrc =
+  const displaySrc = serverAvatarUrl ?? localPreviewUrl;
-    typeof state.communityAvatarUrl === "string" &&
-    state.communityAvatarUrl.trim().length > 0
-      ? state.communityAvatarUrl.trim()
-      : localPreviewUrl;
   const hasPreview = typeof displaySrc === "string" && displaySrc.length > 0;
 
   return (
@@ -161,7 +213,7 @@ export function CommunityUploadScreen() {
           type="file"
           className="sr-only"
           tabIndex={-1}
-          accept="image/jpeg,image/png,image/webp,image/gif"
+          accept={COMMUNITY_AVATAR_ACCEPT}
           aria-label={u.hintText}
           onChange={handleFileChange}
         />

app/api/uploads/route.ts

@@ -14,13 +14,40 @@ import { saveCreateFlowUpload } from "../../../lib/server/uploads/saveCreateFlow
 import { getUploadRootFromEnv } from "../../../lib/server/uploads/uploadRoot";
 import {
   CREATE_FLOW_UPLOAD_MAX_BYTES,
+  maxBytesForPurpose,
   type CreateFlowUploadPurpose,
 } from "../../../lib/server/uploads/uploadConstants";
+import type { CreateFlowUploadValidationReason } from "../../../lib/create/createFlowUploadValidation";
+
+function asUploadedBlob(value: FormDataEntryValue | null): Blob | null {
+  if (typeof value !== "object" || value === null) return null;
+  const candidate = value as Partial<Blob>;
+  if (typeof candidate.arrayBuffer !== "function") return null;
+  if (typeof candidate.size !== "number") return null;
+  return value as Blob;
+}
 
 function isPurpose(x: string): x is CreateFlowUploadPurpose {
   return x === "communityAvatar" || x === "customMethodAttachment";
 }
 
+function messageForValidationReason(
+  reason: CreateFlowUploadValidationReason,
+): string {
+  switch (reason) {
+    case "empty":
+      return "File is empty.";
+    case "tooLarge":
+      return "File exceeds the maximum allowed size for this upload purpose.";
+    case "svg":
+      return "SVG uploads are not allowed.";
+    case "undecodable":
+      return "File could not be decoded as a valid image.";
+    case "invalidType":
+      return "File type is not allowed for this upload purpose.";
+  }
+}
+
 export const POST = apiRoute("uploads.post", async (request: NextRequest) => {
   if (!isDatabaseConfigured()) {
     return dbUnavailable();
@@ -54,7 +81,7 @@ export const POST = apiRoute("uploads.post", async (request: NextRequest) => {
   }
 
   const purposeRaw = formData.get("purpose");
-  const file = formData.get("file");
+  const file = asUploadedBlob(formData.get("file"));
 
   if (typeof purposeRaw !== "string" || !isPurpose(purposeRaw)) {
     return errorJson(
@@ -64,7 +91,7 @@ export const POST = apiRoute("uploads.post", async (request: NextRequest) => {
     );
   }
 
-  if (!(file instanceof File)) {
+  if (!file) {
     return errorJson(
       "validation_error",
       "Missing `file` field (multipart file).",
@@ -72,21 +99,29 @@ export const POST = apiRoute("uploads.post", async (request: NextRequest) => {
     );
   }
 
-  if (file.size > CREATE_FLOW_UPLOAD_MAX_BYTES) {
+  if (file.size === 0) {
+    return errorJson("validation_error", messageForValidationReason("empty"), 400, {
+      details: { reason: "empty" },
+    });
+  }
+
+  if (
+    file.size > CREATE_FLOW_UPLOAD_MAX_BYTES ||
+    file.size > maxBytesForPurpose(purposeRaw)
+  ) {
     return errorJson(
       "payload_too_large",
-      `File exceeds maximum allowed size (${CREATE_FLOW_UPLOAD_MAX_BYTES} bytes).`,
+      messageForValidationReason("tooLarge"),
       413,
+      { details: { reason: "tooLarge" } },
     );
   }
 
   const buf = Buffer.from(await file.arrayBuffer());
-  const mimeType = file.type || "application/octet-stream";
 
   const saved = await saveCreateFlowUpload({
     purpose: purposeRaw,
     buffer: buf,
-    mimeType,
   });
 
   if ("error" in saved) {
@@ -95,10 +130,20 @@ export const POST = apiRoute("uploads.post", async (request: NextRequest) => {
         "File uploads are not configured (UPLOAD_ROOT is unset).",
       );
     }
+    const reason = saved.reason ?? "invalidType";
+    if (reason === "tooLarge") {
+      return errorJson(
+        "payload_too_large",
+        messageForValidationReason("tooLarge"),
+        413,
+        { details: { reason } },
+      );
+    }
     return errorJson(
       "validation_error",
-      "File type or size is not allowed for this upload purpose.",
+      messageForValidationReason(reason),
       400,
+      { details: { reason } },
     );
   }
 

lib/create/createFlowUploadValidation.ts

@@ -0,0 +1,402 @@
+import type { CreateFlowUploadPurpose } from "./createFlowUploadPurpose";
+
+/** Community avatar cap (bytes). */
+const COMMUNITY_AVATAR_MAX_BYTES = 5 * 1024 * 1024;
+/** Custom-method attachment cap (bytes). */
+const CUSTOM_ATTACHMENT_MAX_BYTES = 10 * 1024 * 1024;
+
+export const COMMUNITY_AVATAR_ACCEPT =
+  "image/jpeg,image/png,image/webp,image/gif";
+export const CUSTOM_ATTACHMENT_ACCEPT = `${COMMUNITY_AVATAR_ACCEPT},application/pdf`;
+
+export type CreateFlowUploadValidationReason =
+  | "empty"
+  | "tooLarge"
+  | "svg"
+  | "invalidType"
+  | "undecodable";
+
+type SniffedUploadKind =
+  | "jpeg"
+  | "png"
+  | "gif"
+  | "webp"
+  | "pdf"
+  | "svg"
+  | "empty"
+  | "unknown";
+
+type CreateFlowUploadRasterKind = "jpeg" | "png" | "gif" | "webp";
+
+type CreateFlowUploadValidationOk = {
+  ok: true;
+  kind: CreateFlowUploadRasterKind | "pdf";
+  mimeType: string;
+};
+
+type CreateFlowUploadValidationFail = {
+  ok: false;
+  reason: CreateFlowUploadValidationReason;
+};
+
+export type CreateFlowUploadValidationResult =
+  | CreateFlowUploadValidationOk
+  | CreateFlowUploadValidationFail;
+
+export class CreateFlowUploadValidationError extends Error {
+  readonly reason: CreateFlowUploadValidationReason;
+
+  constructor(reason: CreateFlowUploadValidationReason) {
+    super(reason);
+    this.name = "CreateFlowUploadValidationError";
+    this.reason = reason;
+  }
+}
+
+const SVG_MIME = new Set(["image/svg+xml", "image/svg"]);
+
+export function maxBytesForPurpose(purpose: CreateFlowUploadPurpose): number {
+  return purpose === "communityAvatar"
+    ? COMMUNITY_AVATAR_MAX_BYTES
+    : CUSTOM_ATTACHMENT_MAX_BYTES;
+}
+
+function mimeTypeForSniffedKind(
+  kind: CreateFlowUploadRasterKind | "pdf",
+): string {
+  switch (kind) {
+    case "jpeg":
+      return "image/jpeg";
+    case "png":
+      return "image/png";
+    case "gif":
+      return "image/gif";
+    case "webp":
+      return "image/webp";
+    case "pdf":
+      return "application/pdf";
+  }
+}
+
+export function messageKeyForCreateFlowUploadReason(
+  reason: CreateFlowUploadValidationReason,
+): `errors.${CreateFlowUploadValidationReason}` {
+  return `errors.${reason}`;
+}
+
+function fileLooksLikeSvg(file: File): boolean {
+  const declared = file.type.toLowerCase().split(";")[0]?.trim() ?? "";
+  if (SVG_MIME.has(declared)) return true;
+  return /\.svgz?$/i.test(file.name);
+}
+
+function readU16LE(bytes: Uint8Array, offset: number): number {
+  return bytes[offset]! | (bytes[offset + 1]! << 8);
+}
+
+function readU16BE(bytes: Uint8Array, offset: number): number {
+  return (bytes[offset]! << 8) | bytes[offset + 1]!;
+}
+
+function readU24LE(bytes: Uint8Array, offset: number): number {
+  return (
+    bytes[offset]! | (bytes[offset + 1]! << 8) | (bytes[offset + 2]! << 16)
+  );
+}
+
+function readU32BE(bytes: Uint8Array, offset: number): number {
+  return (
+    ((bytes[offset]! << 24) |
+      (bytes[offset + 1]! << 16) |
+      (bytes[offset + 2]! << 8) |
+      bytes[offset + 3]!) >>>
+    0
+  );
+}
+
+function startsWith(bytes: Uint8Array, offset: number, ascii: string): boolean {
+  if (offset + ascii.length > bytes.length) return false;
+  for (let i = 0; i < ascii.length; i++) {
+    if (bytes[offset + i] !== ascii.charCodeAt(i)) return false;
+  }
+  return true;
+}
+
+function looksLikeSvgBytes(bytes: Uint8Array): boolean {
+  let i = 0;
+  if (
+    bytes.length >= 3 &&
+    bytes[0] === 0xef &&
+    bytes[1] === 0xbb &&
+    bytes[2] === 0xbf
+  ) {
+    i = 3;
+  }
+  while (
+    i < bytes.length &&
+    (bytes[i] === 0x20 ||
+      bytes[i] === 0x09 ||
+      bytes[i] === 0x0d ||
+      bytes[i] === 0x0a)
+  ) {
+    i += 1;
+  }
+  const head = new TextDecoder("utf-8", { fatal: false })
+    .decode(bytes.subarray(i, Math.min(i + 512, bytes.length)))
+    .toLowerCase();
+  return head.includes("<svg") || head.includes("<!doctype svg");
+}
+
+/**
+ * True-type sniff from magic bytes. Raster/PDF signatures win over a later
+ * `<svg` substring so binary comments cannot be classified as SVG.
+ */
+function sniffCreateFlowUploadBytes(
+  bytes: Uint8Array,
+): SniffedUploadKind {
+  if (bytes.length === 0) return "empty";
+  if (bytes.length >= 3 && bytes[0] === 0xff && bytes[1] === 0xd8 && bytes[2] === 0xff) {
+    return "jpeg";
+  }
+  if (
+    bytes.length >= 8 &&
+    bytes[0] === 0x89 &&
+    bytes[1] === 0x50 &&
+    bytes[2] === 0x4e &&
+    bytes[3] === 0x47 &&
+    bytes[4] === 0x0d &&
+    bytes[5] === 0x0a &&
+    bytes[6] === 0x1a &&
+    bytes[7] === 0x0a
+  ) {
+    return "png";
+  }
+  if (startsWith(bytes, 0, "GIF87a") || startsWith(bytes, 0, "GIF89a")) {
+    return "gif";
+  }
+  if (
+    bytes.length >= 12 &&
+    startsWith(bytes, 0, "RIFF") &&
+    startsWith(bytes, 8, "WEBP")
+  ) {
+    return "webp";
+  }
+  if (bytes.length >= 5 && startsWith(bytes, 0, "%PDF-")) {
+    return "pdf";
+  }
+  if (looksLikeSvgBytes(bytes)) return "svg";
+  return "unknown";
+}
+
+function pngDimensions(
+  bytes: Uint8Array,
+): { width: number; height: number } | null {
+  if (bytes.length < 24 || !startsWith(bytes, 12, "IHDR")) return null;
+  const width = readU32BE(bytes, 16);
+  const height = readU32BE(bytes, 20);
+  if (width < 1 || height < 1) return null;
+  return { width, height };
+}
+
+function gifDimensions(
+  bytes: Uint8Array,
+): { width: number; height: number } | null {
+  if (bytes.length < 10) return null;
+  const width = readU16LE(bytes, 6);
+  const height = readU16LE(bytes, 8);
+  if (width < 1 || height < 1) return null;
+  return { width, height };
+}
+
+function jpegDimensions(
+  bytes: Uint8Array,
+): { width: number; height: number } | null {
+  if (bytes.length < 4) return null;
+  let offset = 2;
+  while (offset + 8 < bytes.length) {
+    if (bytes[offset] !== 0xff) {
+      offset += 1;
+      continue;
+    }
+    const marker = bytes[offset + 1]!;
+    if (marker === 0xff) {
+      offset += 1;
+      continue;
+    }
+    if (marker === 0xd8 || marker === 0xd9 || (marker >= 0xd0 && marker <= 0xd7)) {
+      offset += 2;
+      continue;
+    }
+    if (offset + 3 >= bytes.length) return null;
+    const size = readU16BE(bytes, offset + 2);
+    if (size < 2) return null;
+    const isSof =
+      (marker >= 0xc0 && marker <= 0xc3) ||
+      (marker >= 0xc5 && marker <= 0xc7) ||
+      (marker >= 0xc9 && marker <= 0xcb) ||
+      (marker >= 0xcd && marker <= 0xcf);
+    if (isSof) {
+      if (offset + 8 >= bytes.length) return null;
+      const height = readU16BE(bytes, offset + 5);
+      const width = readU16BE(bytes, offset + 7);
+      if (width < 1 || height < 1) return null;
+      return { width, height };
+    }
+    offset += 2 + size;
+  }
+  return null;
+}
+
+function webpDimensions(
+  bytes: Uint8Array,
+): { width: number; height: number } | null {
+  if (bytes.length < 20) return null;
+  if (startsWith(bytes, 12, "VP8X")) {
+    if (bytes.length < 30) return null;
+    const width = readU24LE(bytes, 24) + 1;
+    const height = readU24LE(bytes, 27) + 1;
+    if (width < 1 || height < 1) return null;
+    return { width, height };
+  }
+  if (startsWith(bytes, 12, "VP8L")) {
+    if (bytes.length < 25 || bytes[20] !== 0x2f) return null;
+    const bits =
+      bytes[21]! |
+      (bytes[22]! << 8) |
+      (bytes[23]! << 16) |
+      (bytes[24]! << 24);
+    const width = (bits & 0x3fff) + 1;
+    const height = ((bits >> 14) & 0x3fff) + 1;
+    if (width < 1 || height < 1) return null;
+    return { width, height };
+  }
+  if (startsWith(bytes, 12, "VP8 ")) {
+    if (bytes.length < 30) return null;
+    if (bytes[23] !== 0x9d || bytes[24] !== 0x01 || bytes[25] !== 0x2a) {
+      return null;
+    }
+    const width = readU16LE(bytes, 26) & 0x3fff;
+    const height = readU16LE(bytes, 28) & 0x3fff;
+    if (width < 1 || height < 1) return null;
+    return { width, height };
+  }
+  return null;
+}
+
+function rasterDimensions(
+  kind: CreateFlowUploadRasterKind,
+  bytes: Uint8Array,
+): { width: number; height: number } | null {
+  switch (kind) {
+    case "png":
+      return pngDimensions(bytes);
+    case "gif":
+      return gifDimensions(bytes);
+    case "jpeg":
+      return jpegDimensions(bytes);
+    case "webp":
+      return webpDimensions(bytes);
+  }
+}
+
+function purposeAllowsKind(
+  purpose: CreateFlowUploadPurpose,
+  kind: CreateFlowUploadRasterKind | "pdf",
+): boolean {
+  if (kind === "pdf") return purpose === "customMethodAttachment";
+  return true;
+}
+
+/**
+ * Size, true-type, SVG, and header-decode checks shared by the browser and
+ * `POST /api/uploads`. Call {@link validateCreateFlowUploadFile} on the client
+ * so oversized files are rejected before they are read into memory.
+ */
+export function validateCreateFlowUploadBytes(
+  purpose: CreateFlowUploadPurpose,
+  bytes: Uint8Array,
+): CreateFlowUploadValidationResult {
+  if (bytes.length === 0) return { ok: false, reason: "empty" };
+  if (bytes.length > maxBytesForPurpose(purpose)) {
+    return { ok: false, reason: "tooLarge" };
+  }
+
+  const sniffed = sniffCreateFlowUploadBytes(bytes);
+  if (sniffed === "empty") return { ok: false, reason: "empty" };
+  if (sniffed === "svg") return { ok: false, reason: "svg" };
+  if (sniffed === "unknown") return { ok: false, reason: "invalidType" };
+  if (!purposeAllowsKind(purpose, sniffed)) {
+    return { ok: false, reason: "invalidType" };
+  }
+  if (sniffed === "pdf") {
+    return { ok: true, kind: "pdf", mimeType: mimeTypeForSniffedKind("pdf") };
+  }
+
+  if (rasterDimensions(sniffed, bytes) == null) {
+    return { ok: false, reason: "undecodable" };
+  }
+
+  return {
+    ok: true,
+    kind: sniffed,
+    mimeType: mimeTypeForSniffedKind(sniffed),
+  };
+}
+
+async function readBlobBytes(blob: Blob): Promise<Uint8Array> {
+  if (typeof blob.arrayBuffer === "function") {
+    return new Uint8Array(await blob.arrayBuffer());
+  }
+  return new Promise((resolve, reject) => {
+    const reader = new FileReader();
+    reader.onload = () => {
+      resolve(new Uint8Array(reader.result as ArrayBuffer));
+    };
+    reader.onerror = () => {
+      reject(reader.error ?? new Error("FileReader failed"));
+    };
+    reader.readAsArrayBuffer(blob);
+  });
+}
+
+async function decodeRasterInBrowser(
+  bytes: Uint8Array,
+  mimeType: string,
+): Promise<boolean> {
+  if (typeof createImageBitmap !== "function") return true;
+  try {
+    const copy = new Uint8Array(bytes);
+    const bitmap = await createImageBitmap(
+      new Blob([copy], { type: mimeType }),
+    );
+    const ok = bitmap.width > 0 && bitmap.height > 0;
+    bitmap.close();
+    return ok;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Client-side validation: size and SVG name/type before reading bytes, then
+ * the same sniff/decode path as the server, plus `createImageBitmap` when
+ * available.
+ */
+export async function validateCreateFlowUploadFile(
+  file: File,
+  purpose: CreateFlowUploadPurpose,
+): Promise<CreateFlowUploadValidationResult> {
+  if (file.size === 0) return { ok: false, reason: "empty" };
+  if (file.size > maxBytesForPurpose(purpose)) {
+    return { ok: false, reason: "tooLarge" };
+  }
+  if (fileLooksLikeSvg(file)) return { ok: false, reason: "svg" };
+
+  const bytes = await readBlobBytes(file);
+  const result = validateCreateFlowUploadBytes(purpose, bytes);
+  if (result.ok === false || result.kind === "pdf") return result;
+
+  const decoded = await decodeRasterInBrowser(bytes, result.mimeType);
+  if (!decoded) return { ok: false, reason: "undecodable" };
+  return result;
+}

lib/create/pendingCommunityAvatarUpload.ts

@@ -22,7 +22,20 @@ function openDb(): Promise<IDBDatabase> {
   });
 }
 
+function coerceToFile(value: unknown): File | null {
+  if (value instanceof File) return value;
+  if (typeof Blob !== "undefined" && value instanceof Blob) {
+    return new File([value], "community-avatar", {
+      type: value.type || "application/octet-stream",
+    });
+  }
+  return null;
+}
+
 export async function storePendingCommunityAvatarFile(file: File): Promise<void> {
+  if (typeof indexedDB === "undefined") {
+    throw new Error("indexedDB is not available");
+  }
   const db = await openDb();
   try {
     await new Promise<void>((resolve, reject) => {
@@ -38,6 +51,7 @@ export async function storePendingCommunityAvatarFile(file: File): Promise<void>
 
 /** Read staged file without removing it (caller clears after successful upload). */
 export async function readPendingCommunityAvatarFile(): Promise<File | null> {
+  if (typeof indexedDB === "undefined") return null;
   const db = await openDb();
   try {
     return await new Promise<File | null>((resolve, reject) => {
@@ -45,8 +59,7 @@ export async function readPendingCommunityAvatarFile(): Promise<File | null> {
       tx.onerror = () => reject(tx.error ?? new Error("indexedDB read failed"));
       const getReq = tx.objectStore(STORE).get(KEY);
       getReq.onsuccess = () => {
-        const v = getReq.result;
+        resolve(coerceToFile(getReq.result));
-        resolve(v instanceof File ? v : null);
       };
       getReq.onerror = () => reject(getReq.error);
     });

lib/create/uploadToServer.ts

@@ -1,4 +1,10 @@
 import type { CreateFlowUploadPurpose } from "./createFlowUploadPurpose";
+import {
+  CreateFlowUploadValidationError,
+  messageKeyForCreateFlowUploadReason,
+  validateCreateFlowUploadFile,
+  type CreateFlowUploadValidationReason,
+} from "./createFlowUploadValidation";
 
 export type UploadToServerResult = {
   url: string;
@@ -7,6 +13,20 @@ export type UploadToServerResult = {
   byteLength: number;
 };
 
+const VALIDATION_REASONS = new Set<CreateFlowUploadValidationReason>([
+  "empty",
+  "tooLarge",
+  "svg",
+  "invalidType",
+  "undecodable",
+]);
+
+function reasonFromUnknown(value: unknown): CreateFlowUploadValidationReason | null {
+  return typeof value === "string" && VALIDATION_REASONS.has(value as CreateFlowUploadValidationReason)
+    ? (value as CreateFlowUploadValidationReason)
+    : null;
+}
+
 /**
  * Authenticated multipart upload to `POST /api/uploads`.
  * Caller must have a session cookie (same-origin fetch).
@@ -15,6 +35,11 @@ export async function uploadCreateFlowFile(
   file: File,
   purpose: CreateFlowUploadPurpose,
 ): Promise<UploadToServerResult> {
+  const validated = await validateCreateFlowUploadFile(file, purpose);
+  if (validated.ok === false) {
+    throw new CreateFlowUploadValidationError(validated.reason);
+  }
+
   const formData = new FormData();
   formData.append("purpose", purpose);
   formData.append("file", file);
@@ -36,14 +61,24 @@ export async function uploadCreateFlowFile(
     if (body && typeof body === "object" && "error" in body) {
       const e = (body as {
         error?: { message?: string; code?: string };
+        details?: { reason?: unknown };
       }).error;
-      if (!e) return { message: null as string | null, code: null as string | null };
+      const details = (body as { details?: { reason?: unknown } }).details;
+      const reason = reasonFromUnknown(details?.reason);
+      if (!e) {
+        return {
+          message: null as string | null,
+          code: null as string | null,
+          reason,
+        };
+      }
       return {
         message: typeof e.message === "string" ? e.message : null,
         code: typeof e.code === "string" ? e.code : null,
+        reason,
       };
     }
-    return { message: null, code: null };
+    return { message: null, code: null, reason: null };
   })();
 
   if (!res.ok) {
@@ -54,7 +89,7 @@ export async function uploadCreateFlowFile(
           ? "UNAUTHORIZED"
           : "UPLOAD_FAILED";
     const code = errParts.code ?? errParts.message ?? fallback;
-    throw new UploadToServerError(res.status, code);
+    throw new UploadToServerError(res.status, code, errParts.reason);
   }
 
   const data = body as {
@@ -83,11 +118,34 @@ export async function uploadCreateFlowFile(
 export class UploadToServerError extends Error {
   readonly status: number;
   readonly code: string;
+  readonly reason: CreateFlowUploadValidationReason | null;
 
-  constructor(status: number, code: string) {
+  constructor(
+    status: number,
+    code: string,
+    reason: CreateFlowUploadValidationReason | null = null,
+  ) {
     super(code);
     this.name = "UploadToServerError";
     this.status = status;
     this.code = code;
+    this.reason = reason;
   }
 }
+
+export function createFlowUploadFailureMessageKey(err: unknown): string {
+  if (err instanceof CreateFlowUploadValidationError) {
+    return messageKeyForCreateFlowUploadReason(err.reason);
+  }
+  if (err instanceof UploadToServerError) {
+    if (err.reason) return messageKeyForCreateFlowUploadReason(err.reason);
+    if (err.status === 413) return "errors.tooLarge";
+    if (err.status === 401) return "errors.unauthorized";
+    if (err.code === "server_misconfigured") {
+      return "errors.misconfigured";
+    }
+  }
+  return "errors.generic";
+}
+
+export { CreateFlowUploadValidationError };

lib/server/uploads/saveCreateFlowUpload.ts

@@ -2,12 +2,12 @@ import { writeFile } from "node:fs/promises";
 import path from "node:path";
 import { randomUUID } from "node:crypto";
 import type { CreateFlowUploadPurpose } from "./uploadConstants";
-import {
+import { extensionForMime } from "./uploadConstants";
-  extensionForMime,
-  isAllowedMime,
-  maxBytesForPurpose,
-} from "./uploadConstants";
 import { ensureUploadRootExists, getUploadRootFromEnv } from "./uploadRoot";
+import {
+  validateCreateFlowUploadBytes,
+  type CreateFlowUploadValidationReason,
+} from "../../create/createFlowUploadValidation";
 
 export type SaveCreateFlowUploadResult = {
   /** Filename stem (UUID) without extension — used in GET URL. */
@@ -18,30 +18,32 @@ export type SaveCreateFlowUploadResult = {
   byteLength: number;
 };
 
+export type SaveCreateFlowUploadFailure = {
+  error: "misconfigured" | "validation";
+  reason?: CreateFlowUploadValidationReason;
+};
+
 /**
  * Writes bytes under `UPLOAD_ROOT/{id}{ext}` and returns a stable app URL path.
+ * Trusts sniffed bytes, not the client-declared MIME type.
  */
 export async function saveCreateFlowUpload(params: {
   purpose: CreateFlowUploadPurpose;
   buffer: Buffer;
-  /** Declared MIME from the client `File.type` (validated server-side). */
+}): Promise<SaveCreateFlowUploadResult | SaveCreateFlowUploadFailure> {
-  mimeType: string;
-}): Promise<SaveCreateFlowUploadResult | { error: "misconfigured" | "validation" }> {
   const root = getUploadRootFromEnv();
   if (!root) {
     return { error: "misconfigured" };
   }
 
-  const { purpose, buffer, mimeType } = params;
+  const { purpose, buffer } = params;
-  if (buffer.length > maxBytesForPurpose(purpose)) {
+  const validated = validateCreateFlowUploadBytes(purpose, buffer);
-    return { error: "validation" };
+  if (validated.ok === false) {
-  }
+    return { error: "validation", reason: validated.reason };
-  if (!isAllowedMime(purpose, mimeType)) {
-    return { error: "validation" };
   }
 
   const id = randomUUID();
-  const ext = extensionForMime(mimeType);
+  const ext = extensionForMime(validated.mimeType);
   const fileName = `${id}${ext}`;
   const absolutePath = path.join(root, fileName);
 
@@ -51,7 +53,7 @@ export async function saveCreateFlowUpload(params: {
   return {
     id,
     urlPath: `/api/uploads/${id}`,
-    mimeType: mimeType.toLowerCase().split(";")[0]?.trim() ?? "application/octet-stream",
+    mimeType: validated.mimeType,
     byteLength: buffer.length,
   };
 }

lib/server/uploads/uploadConstants.ts

@@ -1,39 +1,10 @@
-import type { CreateFlowUploadPurpose } from "../../create/createFlowUploadPurpose";
+export type { CreateFlowUploadPurpose } from "../../create/createFlowUploadPurpose";
-
-export type { CreateFlowUploadPurpose };
 export { CREATE_FLOW_UPLOAD_PURPOSES } from "../../create/createFlowUploadPurpose";
+export { maxBytesForPurpose } from "../../create/createFlowUploadValidation";
 
 /** Max body size for multipart upload (bytes). */
 export const CREATE_FLOW_UPLOAD_MAX_BYTES = 12 * 1024 * 1024;
 
-const COMMUNITY_MAX = 5 * 1024 * 1024;
-const CUSTOM_MAX = 10 * 1024 * 1024;
-
-const IMAGE_MIMES = new Set([
-  "image/jpeg",
-  "image/png",
-  "image/webp",
-  "image/gif",
-]);
-
-const CUSTOM_EXTRA_MIMES = new Set(["application/pdf"]);
-
-export function maxBytesForPurpose(purpose: CreateFlowUploadPurpose): number {
-  return purpose === "communityAvatar" ? COMMUNITY_MAX : CUSTOM_MAX;
-}
-
-export function isAllowedMime(
-  purpose: CreateFlowUploadPurpose,
-  mime: string,
-): boolean {
-  const m = mime.toLowerCase().split(";")[0]?.trim() ?? "";
-  if (IMAGE_MIMES.has(m)) return true;
-  if (purpose === "customMethodAttachment" && CUSTOM_EXTRA_MIMES.has(m)) {
-    return true;
-  }
-  return false;
-}
-
 /** Extension including dot, from normalized mime (lowercase). */
 export function extensionForMime(mime: string): string {
   const m = mime.toLowerCase().split(";")[0]?.trim() ?? "";

messages/en/create/upload.json

@@ -1,9 +1,13 @@
 {
   "errors": {
     "generic": "Something went wrong while uploading. Try again.",
-    "tooLarge": "That file is too large. Try a smaller image or PDF.",
+    "tooLarge": "That file is too large. Community photos can be up to 5 MB; attachments up to 10 MB.",
     "unauthorized": "Sign in to upload files. Use Save progress if you started without an account.",
-    "misconfigured": "Uploads are not available on this server yet."
+    "misconfigured": "Uploads are not available on this server yet.",
+    "empty": "That file is empty. Choose a file that has content.",
+    "svg": "SVG files aren't supported. Export a JPEG, PNG, WebP, or GIF instead.",
+    "invalidType": "That file isn't a supported image (or PDF for attachments). Renaming the extension isn't enough.",
+    "undecodable": "We couldn't read that image. It may be damaged — try another file."
   },
   "uploading": "Uploading…"
 }

tests/components/UploadPage.test.tsx

@@ -1,9 +1,71 @@
-import { describe, it, expect } from "vitest";
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
-import { renderWithProviders as render, screen } from "../utils/test-utils";
+import {
+  renderWithProviders as render,
+  screen,
+  fireEvent,
+  cleanup,
+} from "../utils/test-utils";
+import userEvent from "@testing-library/user-event";
 import "@testing-library/jest-dom/vitest";
 import { CommunityUploadScreen } from "../../app/(app)/create/screens/upload/CommunityUploadScreen";
+import {
+  clearPendingCommunityAvatarFile,
+  readPendingCommunityAvatarFile,
+  storePendingCommunityAvatarFile,
+} from "../../lib/create/pendingCommunityAvatarUpload";
+import { fetchAuthSession } from "../../lib/create/api";
+import messages from "../../messages/en/index";
+
+vi.mock("../../lib/create/pendingCommunityAvatarUpload", () => ({
+  readPendingCommunityAvatarFile: vi.fn().mockResolvedValue(null),
+  storePendingCommunityAvatarFile: vi.fn().mockResolvedValue(undefined),
+  clearPendingCommunityAvatarFile: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("../../lib/create/api", () => ({
+  fetchAuthSession: vi.fn().mockResolvedValue({ user: null }),
+}));
+
+const pngBytes = Uint8Array.from(
+  Buffer.from(
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg==",
+    "base64",
+  ),
+);
+
+const copy = messages.create.community.communityUpload;
+const uploadErrors = messages.create.upload.errors;
+
+function fileInput(): HTMLInputElement {
+  const input = document.querySelector('input[type="file"]');
+  expect(input).toBeInstanceOf(HTMLInputElement);
+  return input as HTMLInputElement;
+}
 
 describe("CommunityUploadScreen", () => {
+  beforeEach(() => {
+    vi.mocked(readPendingCommunityAvatarFile).mockResolvedValue(null);
+    vi.mocked(storePendingCommunityAvatarFile).mockResolvedValue(undefined);
+    vi.mocked(clearPendingCommunityAvatarFile).mockResolvedValue(undefined);
+    vi.mocked(fetchAuthSession).mockResolvedValue({ user: null });
+    Object.defineProperty(URL, "createObjectURL", {
+      value: vi.fn(() => "blob:pending-avatar"),
+      writable: true,
+      configurable: true,
+    });
+    Object.defineProperty(URL, "revokeObjectURL", {
+      value: vi.fn(),
+      writable: true,
+      configurable: true,
+    });
+  });
+
+  afterEach(() => {
+    cleanup();
+    Reflect.deleteProperty(URL, "createObjectURL");
+    Reflect.deleteProperty(URL, "revokeObjectURL");
+  });
+
   it("renders HeaderLockup", () => {
     render(<CommunityUploadScreen />);
     expect(
@@ -22,4 +84,76 @@ describe("CommunityUploadScreen", () => {
       ),
     ).toBeInTheDocument();
   });
+
+  it("restores a pending IndexedDB photo and lets the user remove it", async () => {
+    const user = userEvent.setup();
+    const pending = new File([pngBytes], "avatar.png", { type: "image/png" });
+    vi.mocked(readPendingCommunityAvatarFile).mockResolvedValue(pending);
+
+    render(<CommunityUploadScreen />);
+
+    const preview = await screen.findByRole("img", { name: copy.previewAlt });
+    expect(preview).toHaveAttribute("src", "blob:pending-avatar");
+
+    const remove = screen.getByRole("button", {
+      name: copy.clearPendingUploadAriaLabel,
+    });
+    await user.click(remove);
+
+    expect(clearPendingCommunityAvatarFile).toHaveBeenCalled();
+    expect(screen.getByRole("button", { name: "Upload" })).toBeInTheDocument();
+    expect(
+      screen.queryByRole("img", { name: copy.previewAlt }),
+    ).not.toBeInTheDocument();
+  });
+
+  it("rejects an empty file with a clear error and does not stage it", async () => {
+    render(<CommunityUploadScreen />);
+    await screen.findByText(copy.signInToUploadNote);
+
+    fireEvent.change(fileInput(), {
+      target: { files: [new File([], "empty.png", { type: "image/png" })] },
+    });
+
+    expect(await screen.findByRole("alert")).toHaveTextContent(
+      uploadErrors.empty,
+    );
+    expect(storePendingCommunityAvatarFile).not.toHaveBeenCalled();
+  });
+
+  it("rejects a text file renamed as PNG", async () => {
+    render(<CommunityUploadScreen />);
+    await screen.findByText(copy.signInToUploadNote);
+
+    fireEvent.change(fileInput(), {
+      target: {
+        files: [new File(["hello"], "photo.png", { type: "image/png" })],
+      },
+    });
+
+    expect(await screen.findByRole("alert")).toHaveTextContent(
+      uploadErrors.invalidType,
+    );
+    expect(storePendingCommunityAvatarFile).not.toHaveBeenCalled();
+  });
+
+  it("rejects SVG uploads", async () => {
+    render(<CommunityUploadScreen />);
+    await screen.findByText(copy.signInToUploadNote);
+
+    fireEvent.change(fileInput(), {
+      target: {
+        files: [
+          new File(
+            ['<svg xmlns="http://www.w3.org/2000/svg"></svg>'],
+            "photo.png",
+            { type: "image/png" },
+          ),
+        ],
+      },
+    });
+
+    expect(await screen.findByRole("alert")).toHaveTextContent(uploadErrors.svg);
+    expect(storePendingCommunityAvatarFile).not.toHaveBeenCalled();
+  });
 });

tests/unit/createFlowUploadConstants.test.ts

@@ -1,7 +1,6 @@
 import { describe, expect, it } from "vitest";
 import {
   extensionForMime,
-  isAllowedMime,
   isValidUploadFileId,
   maxBytesForPurpose,
 } from "../../lib/server/uploads/uploadConstants";
@@ -12,18 +11,6 @@ describe("createFlow upload constants", () => {
     expect(maxBytesForPurpose("customMethodAttachment")).toBe(10 * 1024 * 1024);
   });
 
-  it("isAllowedMime allows images for both purposes", () => {
-    expect(isAllowedMime("communityAvatar", "image/png")).toBe(true);
-    expect(isAllowedMime("customMethodAttachment", "image/jpeg")).toBe(true);
-  });
-
-  it("isAllowedMime allows pdf only for customMethodAttachment", () => {
-    expect(isAllowedMime("communityAvatar", "application/pdf")).toBe(false);
-    expect(isAllowedMime("customMethodAttachment", "application/pdf")).toBe(
-      true,
-    );
-  });
-
   it("extensionForMime maps common types", () => {
     expect(extensionForMime("image/png")).toBe(".png");
     expect(extensionForMime("image/jpeg")).toBe(".jpg");

tests/unit/createFlowUploadValidation.test.ts

@@ -0,0 +1,164 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+  CreateFlowUploadValidationError,
+  maxBytesForPurpose,
+  validateCreateFlowUploadBytes,
+  validateCreateFlowUploadFile,
+} from "../../lib/create/createFlowUploadValidation";
+import { uploadCreateFlowFile } from "../../lib/create/uploadToServer";
+
+/** 1×1 PNG (valid IHDR). */
+const PNG_1X1 = Uint8Array.from(
+  Buffer.from(
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg==",
+    "base64",
+  ),
+);
+
+/** SOF0 1×1 JPEG (header-decodable; not necessarily a complete scan). */
+const JPEG_1X1_SOF = Uint8Array.from([
+  0xff, 0xd8, 0xff, 0xc0, 0x00, 0x0b, 0x08, 0x00, 0x01, 0x00, 0x01, 0x01, 0x01,
+  0x11, 0x00, 0xff, 0xd9,
+]);
+
+const GIF_1X1 = Uint8Array.from([
+  0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+  0x3b,
+]);
+
+/** VP8X 1×1 WebP. */
+const WEBP_1X1 = Uint8Array.from([
+  0x52, 0x49, 0x46, 0x46, 0x16, 0x00, 0x00, 0x00, 0x57, 0x45, 0x42, 0x50, 0x56,
+  0x50, 0x38, 0x58, 0x0a, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+  0x00, 0x00, 0x00, 0x00,
+]);
+
+const PDF_STUB = new TextEncoder().encode("%PDF-1.4\n%%EOF\n");
+
+const SVG_MARKUP = new TextEncoder().encode(
+  '<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"></svg>',
+);
+
+const XML_SVG = new TextEncoder().encode(
+  '<?xml version="1.0" encoding="UTF-8"?><svg xmlns="http://www.w3.org/2000/svg"/>',
+);
+
+describe("validateCreateFlowUploadBytes", () => {
+  it("accepts a real PNG as a community avatar", () => {
+    const result = validateCreateFlowUploadBytes("communityAvatar", PNG_1X1);
+    expect(result).toEqual({
+      ok: true,
+      kind: "png",
+      mimeType: "image/png",
+    });
+  });
+
+  it("accepts JPEG, GIF, and WebP headers", () => {
+    expect(validateCreateFlowUploadBytes("communityAvatar", JPEG_1X1_SOF).ok).toBe(
+      true,
+    );
+    expect(validateCreateFlowUploadBytes("communityAvatar", GIF_1X1).ok).toBe(
+      true,
+    );
+    expect(validateCreateFlowUploadBytes("communityAvatar", WEBP_1X1).ok).toBe(
+      true,
+    );
+  });
+
+  it("rejects an empty buffer", () => {
+    expect(validateCreateFlowUploadBytes("communityAvatar", new Uint8Array())).toEqual(
+      { ok: false, reason: "empty" },
+    );
+  });
+
+  it("rejects a file larger than the purpose cap", () => {
+    const oversized = new Uint8Array(maxBytesForPurpose("communityAvatar") + 1);
+    oversized.set(PNG_1X1.subarray(0, 8), 0);
+    expect(validateCreateFlowUploadBytes("communityAvatar", oversized)).toEqual({
+      ok: false,
+      reason: "tooLarge",
+    });
+    expect(maxBytesForPurpose("customMethodAttachment")).toBeGreaterThan(
+      maxBytesForPurpose("communityAvatar"),
+    );
+  });
+
+  it("rejects text spoofed as PNG", () => {
+    expect(
+      validateCreateFlowUploadBytes(
+        "communityAvatar",
+        new TextEncoder().encode("hello world"),
+      ),
+    ).toEqual({ ok: false, reason: "invalidType" });
+  });
+
+  it("rejects SVG markup, including xml-prefixed SVG", () => {
+    expect(validateCreateFlowUploadBytes("communityAvatar", SVG_MARKUP)).toEqual(
+      { ok: false, reason: "svg" },
+    );
+    expect(validateCreateFlowUploadBytes("customMethodAttachment", XML_SVG)).toEqual(
+      { ok: false, reason: "svg" },
+    );
+  });
+
+  it("rejects a PNG signature that cannot be decoded", () => {
+    const truncated = PNG_1X1.subarray(0, 8);
+    expect(validateCreateFlowUploadBytes("communityAvatar", truncated)).toEqual({
+      ok: false,
+      reason: "undecodable",
+    });
+  });
+
+  it("rejects PDF for community avatars and allows it for attachments", () => {
+    expect(validateCreateFlowUploadBytes("communityAvatar", PDF_STUB)).toEqual({
+      ok: false,
+      reason: "invalidType",
+    });
+    expect(validateCreateFlowUploadBytes("customMethodAttachment", PDF_STUB)).toEqual(
+      {
+        ok: true,
+        kind: "pdf",
+        mimeType: "application/pdf",
+      },
+    );
+  });
+});
+
+describe("validateCreateFlowUploadFile", () => {
+  it("rejects empty files before reading bytes", async () => {
+    const file = new File([], "empty.png", { type: "image/png" });
+    await expect(
+      validateCreateFlowUploadFile(file, "communityAvatar"),
+    ).resolves.toEqual({ ok: false, reason: "empty" });
+  });
+
+  it("rejects SVG by name even when the MIME is spoofed", async () => {
+    const file = new File([PNG_1X1], "logo.svg", { type: "image/png" });
+    await expect(
+      validateCreateFlowUploadFile(file, "communityAvatar"),
+    ).resolves.toEqual({ ok: false, reason: "svg" });
+  });
+
+  it("rejects a 17MB file by size without treating it as a type error", async () => {
+    const file = new File([new Uint8Array([0x89, 0x50])], "huge.png", {
+      type: "image/png",
+    });
+    Object.defineProperty(file, "size", { value: 17 * 1024 * 1024 });
+    await expect(
+      validateCreateFlowUploadFile(file, "communityAvatar"),
+    ).resolves.toEqual({ ok: false, reason: "tooLarge" });
+  });
+});
+
+describe("uploadCreateFlowFile", () => {
+  it("does not POST when client validation fails", async () => {
+    const fetchMock = vi.fn();
+    vi.stubGlobal("fetch", fetchMock);
+    const file = new File(["not an image"], "photo.png", { type: "image/png" });
+    await expect(
+      uploadCreateFlowFile(file, "communityAvatar"),
+    ).rejects.toBeInstanceOf(CreateFlowUploadValidationError);
+    expect(fetchMock).not.toHaveBeenCalled();
+    vi.unstubAllGlobals();
+  });
+});

tests/unit/saveCreateFlowUpload.test.ts

@@ -0,0 +1,57 @@
+import { mkdtemp, readdir } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const getUploadRootFromEnvMock = vi.fn();
+let uploadRoot: string | null = null;
+
+vi.mock("../../lib/server/uploads/uploadRoot", async () => {
+  const actual = await vi.importActual<
+    typeof import("../../lib/server/uploads/uploadRoot")
+  >("../../lib/server/uploads/uploadRoot");
+  return {
+    ...actual,
+    getUploadRootFromEnv: () => getUploadRootFromEnvMock(),
+  };
+});
+
+import { saveCreateFlowUpload } from "../../lib/server/uploads/saveCreateFlowUpload";
+
+const PNG_1X1 = Buffer.from(
+  "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg==",
+  "base64",
+);
+
+describe("saveCreateFlowUpload", () => {
+  beforeEach(async () => {
+    uploadRoot = await mkdtemp(path.join(tmpdir(), "cr-upload-save-"));
+    getUploadRootFromEnvMock.mockReset();
+    getUploadRootFromEnvMock.mockImplementation(() => uploadRoot);
+  });
+
+  afterEach(() => {
+    uploadRoot = null;
+  });
+
+  it("writes a sniffed PNG even when the client declared the wrong MIME", async () => {
+    const saved = await saveCreateFlowUpload({
+      purpose: "communityAvatar",
+      buffer: PNG_1X1,
+    });
+    expect("error" in saved).toBe(false);
+    if ("error" in saved) return;
+    expect(saved.mimeType).toBe("image/png");
+    const files = await readdir(uploadRoot!);
+    expect(files).toEqual([`${saved.id}.png`]);
+  });
+
+  it("rejects spoofed bytes without writing", async () => {
+    const saved = await saveCreateFlowUpload({
+      purpose: "communityAvatar",
+      buffer: Buffer.from("not an image"),
+    });
+    expect(saved).toEqual({ error: "validation", reason: "invalidType" });
+    expect(await readdir(uploadRoot!)).toEqual([]);
+  });
+});

tests/unit/uploadsPostRoute.test.ts

@@ -27,6 +27,7 @@ function multipartRequest(opts: {
   purpose?: string;
   fileName?: string;
   fileContent?: string;
+  contentType?: string;
 }): NextRequest {
   const boundary = "----VitestBoundary";
   const parts: string[] = [];
@@ -36,8 +37,9 @@ function multipartRequest(opts: {
     );
   }
   if (opts.fileName && opts.fileContent !== undefined) {
+    const contentType = opts.contentType ?? "image/png";
     parts.push(
-      `--${boundary}\r\nContent-Disposition: form-data; name="file"; filename="${opts.fileName}"\r\nContent-Type: image/png\r\n\r\n${opts.fileContent}\r\n`,
+      `--${boundary}\r\nContent-Disposition: form-data; name="file"; filename="${opts.fileName}"\r\nContent-Type: ${contentType}\r\n\r\n${opts.fileContent}\r\n`,
     );
   }
   parts.push(`--${boundary}--\r\n`);
@@ -56,6 +58,7 @@ beforeEach(() => {
   getUploadRootFromEnvMock.mockReset();
   isDatabaseConfiguredMock.mockReturnValue(true);
   getUploadRootFromEnvMock.mockReturnValue("/tmp/uploads");
+  getSessionUserMock.mockResolvedValue({ id: "u1", email: "a@b.c" });
 });
 
 describe("POST /api/uploads", () => {
@@ -78,7 +81,6 @@ describe("POST /api/uploads", () => {
   });
 
   it("returns 500 when UPLOAD_ROOT is unset", async () => {
-    getSessionUserMock.mockResolvedValueOnce({ id: "u1", email: "a@b.c" });
     getUploadRootFromEnvMock.mockReturnValueOnce(null);
     const res = await POST(
       new NextRequest("https://x.test/api/uploads", { method: "POST" }),
@@ -90,7 +92,6 @@ describe("POST /api/uploads", () => {
   });
 
   it("returns 400 when purpose is missing", async () => {
-    getSessionUserMock.mockResolvedValueOnce({ id: "u1", email: "a@b.c" });
     const res = await POST(
       multipartRequest({ fileName: "avatar.png", fileContent: "x" }),
       undefined,
@@ -99,4 +100,56 @@ describe("POST /api/uploads", () => {
     const body = (await res.json()) as { error: { code: string } };
     expect(body.error.code).toBe("validation_error");
   });
+
+  it("returns 400 for an empty file", async () => {
+    const res = await POST(
+      multipartRequest({
+        purpose: "communityAvatar",
+        fileName: "empty.png",
+        fileContent: "",
+      }),
+      undefined,
+    );
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as {
+      error: { code: string };
+      details?: { reason?: string };
+    };
+    expect(body.error.code).toBe("validation_error");
+    expect(body.details?.reason).toBe("empty");
+  });
+
+  it("returns 400 for SVG even when named .png", async () => {
+    const res = await POST(
+      multipartRequest({
+        purpose: "communityAvatar",
+        fileName: "photo.png",
+        fileContent: '<svg xmlns="http://www.w3.org/2000/svg"></svg>',
+        contentType: "image/png",
+      }),
+      undefined,
+    );
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as {
+      details?: { reason?: string };
+    };
+    expect(body.details?.reason).toBe("svg");
+  });
+
+  it("returns 400 for a text file spoofed as PNG", async () => {
+    const res = await POST(
+      multipartRequest({
+        purpose: "communityAvatar",
+        fileName: "photo.png",
+        fileContent: "not an image",
+        contentType: "image/png",
+      }),
+      undefined,
+    );
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as {
+      details?: { reason?: string };
+    };
+    expect(body.details?.reason).toBe("invalidType");
+  });
 });