Rename email attachments if an attachment with the same filename already exists.

When I attach screenshots to an issue, I usually do this by email. The images included in the email seem to be named sequentially, presumably by my email client e.g. "image1.jpg". When I go to attach a second screenshot in the same manner, the second image has the same filename. This causes a couple of problems. Any links within comments including the first file now point to the second file. I can't include a thumbnail of the second file inline in a comment. This issue could be resolved by automatically renaming the file on the server to something unique.