Drag & Drop problem if file name contains special characters

In Chrome on OSX if you drag & drop a file with a name that contains umlauts (eg. "über_cool.jpg") then moxie silently fails to get the native file object (``webkitGetAsEntry().file()``).
This seems to happen because of a chrome bug.
The attached code will work around this issue by using ``getAsFile`` on the dataTransfer item.