Files that match the `CoverArtPriority` setting will now be considered
eligible only if their extensions are of an 'image/*' MIME type (e.g.
'.png' for 'image/png', '.jpg' for 'image/jpeg'). This prevents matching
files that will likely not be valid during display.
In addition to the above, code for returning the cover image file from
scanned data will also check against the MIME type for the path stored,
instead of attempting to re-trace `CoverArtPriority` matches. This
simplifies the code and bypasses a number of edge-cases related to
inconsistent matching.
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