Add an image mask file type

[bfirsh]

Just wanted to demonstrate what it might be like to define richer file types in Cog that would affect the user interface.

I'm also imagining ImageFile that sets type to be image/*, and some Path equivalents. Open to better ideas for schema design, but I'm eager to just get this out rather than get lost in schema design. We can always change it by just picking a different key, or whatever.

[zeke]

Related: https://github.com/replicate/cog/issues/1341