fix(media): add missing MIME type mappings for audio/video files

Add mappings for audio/x-m4a, audio/mp4, and video/quicktime to ensure
media files sent as documents are saved with proper extensions, enabling
automatic transcription/analysis tools to work correctly.

- audio/x-m4a → .m4a
- audio/mp4 → .m4a
- video/quicktime → .mov

Also adds comprehensive test coverage for extensionForMime().

src/media/mime.ts

@@ -13,7 +13,10 @@ const EXT_BY_MIME: Record<string, string> = {
   "image/gif": ".gif",
   "audio/ogg": ".ogg",
   "audio/mpeg": ".mp3",
+  "audio/x-m4a": ".m4a",
+  "audio/mp4": ".m4a",
   "video/mp4": ".mp4",
+  "video/quicktime": ".mov",
   "application/pdf": ".pdf",
   "application/json": ".json",
   "application/zip": ".zip",