This doesn't depend on multimedia backend, can be reproduced with "windows", "darwin", "ffmpeg".

Can be reproduced with standard Audio Recorder Example (qtmultimedia/examples/multimedia/audiorecorder).

Steps to reproduce:
1. Build and run standard Audio Recorder Example
2. Select File Container as "MPEG-4 Audio"
3. Select any Output file with "m4a" extension (e.g. test.m4a)
4. Record some audio
5. Check the actually recorded file

Actual results: file with m4a.mp4a extension has been recorded (e.g. test.m4a.mp4a).
Expected results: Requested output file extension should be preserved (it should be m4a in such case).

Note: there was no such problem on Qt 6.7.2 and before, at least with "windows" and "darwin" backends.