At the moment, it seems to be shown whenever an album/a track has more than one piece of cover art. I think this is not entirely useful: at least after saving files, the button should also be shown if the album/track only has a single piece of cover art because this is really the same as an album/track with multiple pieces - if you want to look at the details, just let me do it. But then the behaviour would be different before and after saving, which imho is really weird. Maybe we should show the button all the time if there's at least one piece of cover art?