Fix IE 11 bug with e.dataTransfer.effectAllowed

Internet Explorer 11 throws an exception (SCRIPT65535) when trying to
read the dataTransfer.effectAllowed property of a DragEvent. Wrapping it
in a try-catch allows dragging files from the Chrome download bar
without affecting Internet Explorer.
Merge pull request #449 from kimsey0/master

Fix IE 11 bug with e.dataTransfer.effectAllowed