Check that files really present in dragging data

According to http://www.w3.org/TR/2010/WD-html5-20101019/dnd.html#dom-datatransfer-types we can check that dragged data contain files. If there are no files we should not show drop area.

My use case is as follow.
I need a drag-n-drop file upload but at the same time I need drag-n-drop of another items on the page.
At the moment during dragging of even text the drop file area is shown.
Merge pull request #155 from dmitry-dedukhin/patch-1

Check that files really present in dragging data