Attachments definitely don't need to be turned on for the gallery to work. There is something that's preventing the tmp directory from being populated however. It uses the standard move_uploaded_file function() which should copy it from the real windows temp directory to the ubbthreads tmp directory.
It might be helpful to test and just turn on file attachments in a regular forum and try to attach some files. That way we're not debugging anything gallery related. It should still copy to the tmp directory first while making the post, and then move the attachments to their final location once the post has been submitted.
