#9857 DnD folder with Firefox 31 behaves incorrectly

Can somebody review this PR ?

Change tested with :
- FF31: folder can be dropped, then a message tells folder dropping is not supported
- Chrome 36:  folder can be dropped and uploaded
- IE 11 : folder cannot be dropped at all.

Can we show a more understandable error here? Above it says »HTTP connection problem«, so what about showing »Connection problem, please try again.« in the notification?