New issue by Bumbadawg on void-packages repository

https://github.com/void-linux/void-packages/issues/30486

Description:
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### System

* xuname:  
  *Void 5.11.16_1 x86_64 GenuineIntel notuptodate rrrrmmnFFFF*
* package:  
  * youtube-dl-2021.04.17_1
  * ffmpeg-4.3.2_1

### Expected behavior
`xdg-mime query filetype audio.m4a` should output `audio/mp4`
NB: it works with the same package versions on Arch.

### Actual behavior
`xdg-mime query filetype audio.m4a` should output `video/mp4`

when reusing ffmpeg afterwards to try to fix the MIME, this is what i get 
`ffmpeg -i in.m4a -acodec copy out.m4a`
`xdg-mime query filetype audio.m4a` => `audio/x-m4a`

`grep m4a /etc/mime.types` => `audio/mpeg     m2a m3a m4a mp2 mp2a mp3 mpega mpga`

### Steps to reproduce the behavior
`youtube-dl -x --audio-format best [YTlink]`