In the older releases of MantisBT, we used to use the file name as the file name on disk. Then we moved to hashes to avoid conflicts. When moving files between projects there is a change that there is a naming conflict between the two attachments on the source and destination projects. But if we upgrade all attachments to a hash, that will become a very rare possibility. It also cleans up the attachments folders by using consistent naming. Also enabling combining multiple projects to share same folder if that is desired by the user.