M4A audio files are rejected as invalid despite being listed as supported

Description

When attempting to upload an .m4a audio file through the Audio object, Capacities immediately rejects the file as invalid, despite .m4a being explicitly listed as a supported format in both the upload interface and the official documentation.

Environment

• Subscription: Capacities Believer
• File format: .m4a
• File size: 71 MB
• Upload method: Audio → Upload → Select File(s)

Steps to reproduce

1. Open Capacities.
2. Create a new Audio object.
3. Select Upload → Select File(s).
4. Select a valid, playable .m4a file (71 MB).
5. Observe that Capacities immediately rejects the file as invalid, before the upload starts.

Expected behavior

The .m4a file should be accepted and uploaded successfully, as the format is explicitly supported and the file is below the documented 100 MB per-file limit.

Actual behavior

The file is immediately rejected as invalid upon selection, preventing the upload.

Troubleshooting performed

• Confirmed that the original .m4a file is valid and playable.
• Confirmed that the file size (71 MB) is below the documented limit.
• Rebuilt the M4A container using FFmpeg with stream copying (-c:a copy), without re-encoding. The resulting file was also rejected.
• Converted the original audio to MP3 using FFmpeg. The MP3 version uploaded successfully.

Additional information

This suggests a possible issue with M4A file validation, codec/container compatibility, or MIME type detection rather than file corruption, account permissions, or file size limitations.

Impact

Users may be forced to re-encode otherwise valid M4A audio files to MP3 before uploading, introducing an unnecessary conversion step.

Could you investigate whether the current M4A validation logic is incorrectly rejecting certain valid M4A files?