I have a client where this would be valuable, also. DMX collections are organized such that pictures (jpg, tif, etc) are in a different root than other documents, although I am sure not everyone would segregate in this fashion.
In any event, being able to see an auto-generated thumbnail would allow them to maintain both file types via DMX rather than implementing a photo gallery, which would behave quite differently than DMX.
So here is another vote for a future enhancement.
Rob Ralston