Allow dragging image file into select image prompt

Currently in the (new) image interface you have to select a file before uploading it. It would be nice if we could drag a file into the upload tag window instead.

Nb. this would save the user from having to browse to the location with the 'choose file' button.

Use case could be when an image field is added in a template which forces users to change the image through the interface. (Opposite to just dragging an image into the page which would create a new image object)