@pfefferle interesting. is there a concept of mixed-media formats, then? the reason I ask is because my first assumption would have been that Image maps to Image, in particular when it's a single Image. likewise, that Video maps to a single Video where possible. but i suppose that wouldn't completely work with multiple media files or mixed media, would it?
it may be worth thinking about alternative approaches like perhaps an "Album" extension type that is a Collection of Image/Video items?