Disallow uploads via drag-and-drop for non-files.
Disallow uploads via drag-and-drop for non-files.

The drag-and-drop support wasn't picky enough about what was being
dropped. It now requires files.

This also removes duplicate dragleave events.

Reviewed at http://reviews.reviewboard.org/r/4207/

brennie/reviewboard,chipx86/reviewboard,davidt/reviewboard,sgallagher/reviewboard,1tush/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,KnowNo/reviewboard,beol/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,1tush/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,beol/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,custode/reviewboard,reviewboard/reviewboard,1tush/reviewboard,custode/reviewboard,brennie/reviewboard,sgallagher/reviewboard,1tush/reviewboard,brennie/reviewboard,beol/reviewboard,reviewboard/reviewboard,brennie/reviewboard,davidt/reviewboard,chipx86/reviewboard,KnowNo/reviewboard,1tush/reviewboard,sgallagher/reviewboard,bkochendorfer/reviewboard,beol/reviewboard,KnowNo/reviewboard,1tush/reviewboard,1tush/reviewboard,reviewboard/reviewboard,custode/reviewboard