Add AI transcription support for m4a audio files

Currently, the AI transcription service for Aha! Discovery supports mp4 video and mp3 audio files.

I can successfully upload an m4a audio file (e.g. recorded using the Voice Memo app for an in person meeting) to Discovery, but I cannot generate a transcript. As a workaround, I would need to convert the file format outside of Aha!, or generate the transcript in another AI tool and then upload this to Aha!

Add support for transcribing m4a audio files.