feat(media): infer MIME from extension for empty-type uploads (.md, .html)

Browsers report an empty file.type for files with no OS-registered MIME (esp.
.md), which then went up as application/octet-stream and got rejected. Infer
the type from the extension for the text types IIOS allows (md/markdown/html/
htm/txt/csv) so those attachments upload.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/media-api.ts

@@ -14,12 +14,25 @@ export function isImage(mime?: string | null): boolean {
   return !!mime && mime.startsWith("image/");
 }
 
+// Some types (notably .md) have no OS-registered MIME, so the browser reports an empty file.type.
+// Fall back to the extension for the text types IIOS allows, else a generic binary.
+const EXT_MIME: Record<string, string> = {
+  md: "text/markdown", markdown: "text/markdown",
+  html: "text/html", htm: "text/html",
+  txt: "text/plain", csv: "text/csv",
+};
+function mimeForFile(file: File): string {
+  if (file.type) return file.type;
+  const ext = file.name.toLowerCase().split(".").pop() ?? "";
+  return EXT_MIME[ext] ?? "application/octet-stream";
+}
+
 /** Upload a File → { contentRef, mimeType, sizeBytes, filename }. Throws on oversize / failure. */
 export function useUploadAttachment() {
   const { sdk } = useAppShell();
   return useCallback(async (file: File): Promise<UploadedAttachment> => {
     if (file.size > MAX_ATTACHMENT_BYTES) throw new Error("File is too large (max 25 MB).");
-    const mime = file.type || "application/octet-stream";
+    const mime = mimeForFile(file);
     const { objectKey, uploadUrl } = (await sdk.command("crm.media.presignUpload", { mime, sizeBytes: file.size })) as { objectKey: string; uploadUrl: string };
     const res = await fetch(uploadUrl, { method: "PUT", body: file });
     if (!res.ok) throw new Error(`Upload failed (${res.status}).`);