It would be pretty nice it when one attachment has been selected for upload a new attachments box would appear.
On the other hand, considering that part of the UI should be kind of removed in 1.1 due to merging of D&D and the "normal" way I wonder if it make sense to propose it... oh well, it's already typed so let's post it. :P
That does make some sense, to automatically open up a new box in those cases, makes it easier for the user. Not sure how we are going to combine that area (program / js question more than anything else), the pics you posted look a lot better than what is there today.