I find it a disconcerting that when the uploads have completed one is only offered the options to "Cancel" or "Add more files".  Clicking on "Cancel" seems like you are going to stop whatever you were doing which is not the case because "Cancel" only resets the session after the uploads have completed and you really can't cancel anything.  I would like to propose that the "Cancel" button switches to "Done" to give a clearer indication of what the options are.

If one adds more files or a session is in progress then it is appropriate for the "Cancel" to be visible.