change event.draggingFile to event.draggingExternal

change `event.draggingFile` to `event.draggingExternal`

A file isn't the only object you can drag into the viewport. For example, it could be an item on your favorites bar or even a browser tab.

Updated the test pages so the drop zones change to different colors based on if the object being dragged originated inside or outside of the viewport, and if it was outside, whether "Files" is one of its types. (green=inside, pink=external file, beige=other external)