Add button and custom modal flow implemented

lib/server/uploads/resolveUploadedFile.ts

@@ -0,0 +1,59 @@
+import { constants as fsConstants } from "node:fs";
+import { access, readdir } from "node:fs/promises";
+import path from "node:path";
+import { getUploadRootFromEnv } from "./uploadRoot";
+import { isValidUploadFileId } from "./uploadConstants";
+
+export type ResolvedUploadFile = {
+  absolutePath: string;
+  /** MIME inferred from extension (no sidecar file in v1). */
+  contentType: string;
+};
+
+function contentTypeForFilename(fileName: string): string {
+  const ext = path.extname(fileName).toLowerCase();
+  switch (ext) {
+    case ".jpg":
+    case ".jpeg":
+      return "image/jpeg";
+    case ".png":
+      return "image/png";
+    case ".webp":
+      return "image/webp";
+    case ".gif":
+      return "image/gif";
+    case ".pdf":
+      return "application/pdf";
+    default:
+      return "application/octet-stream";
+  }
+}
+
+/**
+ * Resolves `id` (UUID stem) to the single matching file `{id}.*` under UPLOAD_ROOT.
+ * Returns null if missing, ambiguous, or invalid id.
+ */
+export async function resolveUploadedFileById(
+  id: string,
+): Promise<ResolvedUploadFile | null> {
+  if (!isValidUploadFileId(id)) return null;
+  const root = getUploadRootFromEnv();
+  if (!root) return null;
+
+  const entries = await readdir(root);
+  const prefix = `${id}.`;
+  const matches = entries.filter((e) => e.startsWith(prefix));
+  if (matches.length !== 1) return null;
+
+  const absolutePath = path.join(root, matches[0]!);
+  try {
+    await access(absolutePath, fsConstants.R_OK);
+  } catch {
+    return null;
+  }
+
+  return {
+    absolutePath,
+    contentType: contentTypeForFilename(matches[0]!),
+  };
+}