running the example below
dragging some text (not a file) over a drop upload component initially ignores the dragged text
-> OK
then dropping a real file triggers the onUpload event as usual
-> OK
again dragging some text causes an onUpload event resulting in a NPE on the server
-> not expected

http://screencast.com/t/MAwzVhvYn3

Expected results:
A) the onUpload event should only be triggered for really uploaded Files -> dragging/dropping just text should be ignored (most important)

B) when not dragging a file the droptarget should not be highlighted at all (nice to have)
e.g. gmail only creates a highlight when a File is dragged over a new mail, just dragging text does not indicate a possible drop target

<zk>
    <vlayout>
        <image id="img" />
        Upload your hot shot:
        <dropupload maxsize="-1" content="content" detection="browser" onUpload="img.setContent(event.media);" />
    </vlayout>
</zk>