first things first. Attachments are not files in their own right. So dragging to the desktop is probably destined to fail. Nope, I was wrong, dragged an attachment from Thunderbird attachment pane to windows desktop. It was there. So I guess we go to the other hint you offer of zero size. Generally that sort of issue is directly caused by antivirus scanning. When you drag, first the attachment data has to become a file. So a temp file goes into the system temp folder. Then it gets moved or copied really when the drop happens. IF the antivirus is a slug and still thinking about it's actions the copy that get to the drop is minus the file content as it is still locked up by the antivirus. So try disabling it for a bit and see if you still have the same problem.