When dragging a file / text / URL over a long section, the 'Add X here' item may be off the bottom of the screen.

It would be good to add some highlight to the whole of the section that is being dragged over at the moment.

To help with this, the section now gains the class 'dndupload-over' when the drag enters the section (the class is removed again when it leaves). I'm not confident about coming up with some suitable styling, so I am hoping to leave that to someone else to complete.