Some files' embedded art seems to have the mimetype 'image/JPG' instead
of 'image/jpg'. Libav fails to parse those because it matches
case-sensitively.

Use av_strncasecmp() to fix this behaviour.
Signed-off-by: Mohammad Alsaleh <msal@tormail.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>