extract: Be consistent about parameter ordering

The media_art_process() and media_art_process_file() functions should
take arguments in the same order as each other.
extract: Remove wrong MIME type check in media_art_process_file()

The 'mime' parameter describes the image buffer, not the media file,
so it's wrong to fill it in from the media file.
extract: Rewrite media_art_process() to use media_art_process_file()
Merge branch 'process-file-fixes'

Reviewed-By: Carlos Garnacho <carlosg@gnome.org>