I started to compose an email, went to another tab for something else and then found the files and clicked and dragged them over the tab for email. Expected Result: tab gets focused and I can drop my files in my composition. Actual Result: I get the red circle with the slash through it letting me know the action is prohibited. For experimentation purpose I tried just dragging one file and the tab got focus properly, so it seems that it's multiple files that don't like to play. (Notable that I'm on Windows 7 as that could be a factor.)
It at least seems to also happen on Mac.