Add button and custom modal flow implemented

lib/server/uploads/saveCreateFlowUpload.ts

@@ -0,0 +1,57 @@
+import { writeFile } from "node:fs/promises";
+import path from "node:path";
+import { randomUUID } from "node:crypto";
+import type { CreateFlowUploadPurpose } from "./uploadConstants";
+import {
+  extensionForMime,
+  isAllowedMime,
+  maxBytesForPurpose,
+} from "./uploadConstants";
+import { ensureUploadRootExists, getUploadRootFromEnv } from "./uploadRoot";
+
+export type SaveCreateFlowUploadResult = {
+  /** Filename stem (UUID) without extension — used in GET URL. */
+  id: string;
+  /** Full relative URL path for clients, e.g. `/api/uploads/abc`. */
+  urlPath: string;
+  mimeType: string;
+  byteLength: number;
+};
+
+/**
+ * Writes bytes under `UPLOAD_ROOT/{id}{ext}` and returns a stable app URL path.
+ */
+export async function saveCreateFlowUpload(params: {
+  purpose: CreateFlowUploadPurpose;
+  buffer: Buffer;
+  /** Declared MIME from the client `File.type` (validated server-side). */
+  mimeType: string;
+}): Promise<SaveCreateFlowUploadResult | { error: "misconfigured" | "validation" }> {
+  const root = getUploadRootFromEnv();
+  if (!root) {
+    return { error: "misconfigured" };
+  }
+
+  const { purpose, buffer, mimeType } = params;
+  if (buffer.length > maxBytesForPurpose(purpose)) {
+    return { error: "validation" };
+  }
+  if (!isAllowedMime(purpose, mimeType)) {
+    return { error: "validation" };
+  }
+
+  const id = randomUUID();
+  const ext = extensionForMime(mimeType);
+  const fileName = `${id}${ext}`;
+  const absolutePath = path.join(root, fileName);
+
+  await ensureUploadRootExists(root);
+  await writeFile(absolutePath, buffer, { mode: 0o644 });
+
+  return {
+    id,
+    urlPath: `/api/uploads/${id}`,
+    mimeType: mimeType.toLowerCase().split(";")[0]?.trim() ?? "application/octet-stream",
+    byteLength: buffer.length,
+  };
+}