Ensure mp3 type is audio/mpeg in media player

[bjester]

Summary

Ensures that we're passing the correct audio type for .mp3 files as it's type should be audio/mpeg.

Reviewer guidance

1. Import audio content from channel nakav-mafak
2. Verify that audio files play correctly in supported browsers

References

Found during investigating: #7395

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Reviewer Checklist

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[bjester]

Once @rtibbles change here is available, we can replace this with looking up the type in the JSON map.

[jonboiser]

Looks good!

[jonboiser]

Looks good. Just looked up the list here https://www.iana.org/assignments/media-types/media-types.xhtml#audio to make sure that the mp3 -> audio/mpeg mapping was a special case. Other popular audio files seem to be more consistent.