Dragging a folder to Upload Files locks Firefox when upload is attempted.

Only appears to be an issued with Firefox (29.0.1 on Mac OS X) but dragging a folder to the file upload drop zone and then clicking `Upload file(s)` locks the browser.

No errors in the console.
Ok cool, I held off on reviewing it since I wasn't completely sure if it was a copy-paste. I'll have a go at it now.