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.
Maybe you could add a word as to why that is? I'm assuming it's because file size wouldn't be supported on browsers that don't support anything else than iframe transport?