change event.draggingFile to event.draggingExternal

- change event.draggingFile to event.draggingExternal since an object
dragged into the viewport is not necessarily a file
- update test pages to show if it's an external file or another external
object being dragged
Merge pull request #2 from wizard04wsu/drag-event-attributes

change `event.draggingFile` to `event.draggingExternal`