MediaFoundation: remove redundant m4v type

[daschuer]

It is already registered as mp4.
This should fix a debug assertion in https://bugs.launchpad.net/mixxx/+bug/1961623

[Holzhaus]

Not really familiar with the code, but shouldn't we remove the mp4 line instead? AFAIK it's a container format and can contain video or audio. m4a and m4v are distinct.

[daschuer]

This is related to the magic number detection, see:
https://www.garykessler.net/library/file_sigs.html

These files are detected as "video/mp4" and we use the file type "mp4" in Mixxx, which matches the QMimeType::preferredSuffix().

"audio/m4a" are detected explicit.

We also use "mp4" here as well:

• mixxx/src/sources/soundsourceffmpeg.cpp

  Line 375 in 1b688f2

  list.append("mp4");

• mixxx/src/sources/soundsourcecoreaudio.cpp

  Line 36 in 1b688f2

  QStringLiteral("mp4"),

[uklotzde]

Using type "mp4" for the suffix ".m4v" seems to be consistent with the remaining code.