This isn't a bug, its back when we allowed people to input links to files instead of uploading.
People would put in the filename, extention, or an alternate link to the file for some reason, even though it wasn't supposed to be filled out.
Today, it has been disabled for all but admins and I think DLDB keepers because its too confusing, plus people have poor hosts for their files anyway. So we prefer all files uploaded to our server.
Putting in a file upload AND a link would make the download appear twice.
Anyways, this bug has been fixed.