If an earlier message is edited in the overlay and a file is dragged from computer to be "attached" (I am not talking about an image dragged from another webpage), then it highlights the editor as a drop target but does nothing once the file is dropped there. I am not sure if it actually uploads the file or simply discards. I am not sure what can be done about it. Will it be possible to disable the drop target highlighting for attachments but keep it enabled for dropping URLs?
I don't understand what you're reporting. If you try to drag a file to it it will tell you that you can't:
I can't reproduce anything but that for a file. If I drag a link in from another tab, it inserts like I'd expect.
I apologize for this false negative report. In fact when drag stops but the drop action is not released (still keep pressing the mouse key for instance) then the message disappears in a very short time. So to read the message, one really has to keep dragging. And that's why I missed the message in the middle. You can discard this report now.