simplify MediaFile art interface: no type is included
simplify MediaFile art interface: no type is included

The interface no longer specifies the type of the image embedded in the file; it
just returns a bytestring blob. When a type must be stored, it is inferred using
the imghdr module, which shoudl reduce the potential for weird bugs when the
formats don't correspond.

Original: 2e23e8204135353f00917f17b08a8f5344fd9e8a@73c4bed

beetbox/mediafile