MediaFile::hasThumbnail() reports true for music files with art that is not cover art

Some media formats allow for the storage of multiple pieces of artwork tagged with an image type. This includes:

 1. ID3v2's APIC artwork
 2. FLAC pictures
 3. vorbiscomment's METADATA_BLOCK_PICTURE (which is just a base64 encoded FLAC picture).

Thumbnailer will only produce a thumbnail for artwork marked as "front cover" or "other". This means that if we encountered a file with art but no "front cover" art, mediascanner might direct an application to request embedded art (which will fail), when it might be possible to successfully get online artist/album artwork.

How likely this is to affect users is unknown. I don't think we've had any bug reports about this behaviour yet, so files this would effect are either exceedingly rare or don't exist outside of test suites.