.m4a files should use audio/mp4 MIME type, not audio/mpeg

Problem/Motivation

.m4a files should use the audio/mp4 MIME type, not audio/mpeg.

The apache httpd mime.types file provides this mapping: https://github.com/apache/httpd/blob/trunk/docs/conf/mime.types

As does debian: https://salsa.debian.org/debian/media-types/-/blob/master/mime.types

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1004985

Steps to reproduce

If a file field uses "Audio" as the field formatter, and an m4a file is uploaded, the resulting audio player doesn't work on iOS (Safari or Chrome) due to this markup:

<audio controls="controls">
  <source src="/uploads/sample1.m4a" type="audio/mpeg">
</audio>

But the audio player works with type="audio/mp4"

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet