I've got some m4a samples that had jpeg cover art marked as png. Since
these files were supposedly written by iTunes, and other software can
read it (e.g. clementine does), this should be worked around.

Since png has a very simple to detect header, while it's apparently a
real pain to detect jpeg in the general case, try to detect png and
assume jpeg otherwise. Not bothering with bmp, as I have no test case.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>