Autodetect format when transcoding files

Fix #1274 (closed)

This Merge Request includes:

• Tests
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)

This sets audio file format to be detected by ffmpeg, instead of inferred from extension. Extension names don't map 1:1 with inout format names that ffmpeg uses. Despite what the pydub docs say, from_file knows how to detect format automatically instead of always defaulting to mp3.