I'm going to ask a very obvious question, so please no one take offense. I'm not trying to insult anyone's intelligence, it's just that when trying to troubleshoot problems, it's best to cover all your bases rather than have something simple come back to bite you later.
Do the files sent using Mail.app have valid file extensions? I ask, because osTicket checks for allowed file types via their extesions and OS X will often not use file extensions for many common filetypes, unless the user specifically adds one when saving the file.