This is likely not Protobi's issue, but it seem to crop up from time to time.  

Two possibilities for HTML5 Drag-Drop not working on Windows 7.  The first is reported by many to resolve the issue but I have no idea how:

Right-click on a file and hold the mouse button down.
While the the mouse is still pressed hit the Esc key.

The second is to reboot the computer.  If you're like me and reboot the computer only once a month or less that might resolve it.  If you rebooted today, that's clearly not it.