fix(preview): Do not try to parse M3U files as MP3

[paulijar]

Summary

By default, the MP3 audio files get the mimetype audio/mpeg and the M3U and M3U8 playlist files get the mimetype audio/mpegurl. PreviewManager had such a problem that it registered the MP3 preview provider with a regular expression which matched also the M3U files. This caused an error message to be logged on the info (1) level for each M3U file every time a folder with such files was viewed: "Error while getting cover from mp3 file: File /path/to/some/playlist.m3u is not mpeg/audio!".

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

Makes sense, though I wonder if we shouldn't make all patterns more strict by using ^ and $ everywhere. For example in the next line .* is used, but is unnecessary at the moment if the start and end don't matter.

[blizzz]

/backport to stable30

[blizzz]

/backport to stable29

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