Ignore album cover for video files

After a quick look of an issue on iOS, it appears that if a video is stored in a folder with a cover.jpg file (or any other file that matches our cover files patterns), this cover file will be used for the video as well, which is incorrect.

I'm not sure how to address this... we must still accept embedded cover, but probably should reject "album" covers. Maybe the correct fix lies in VLC, and we shouldn't try to fetch a cover when there is a video track?