On the "Upload New" source, Nat (and I) both got a bit confused at the idea of uploading multiple images at once. Before doing anything, there is a horizontal slider which is very unexpected. Second, you upload several images, but it is confusing to think how the right hand inspector relates to the left hand.
Nat wants this converted back to single upload. Move the upload part up to the top. Have the bottom show a preview when uploaded. Have the right hand side work against the single image uploaded on the right.
Also, if you upload something and change your mind, you get stuck in that spot until you reload the page. In general, "Cancel" should likely mean, delete the attachment and clear the "Upload New" panel's state.
Finally, replace the string that says "this folder" when upload is happening to say "this page". People don't think of wikipages as folders.
Another non-small set of feedback changes.