We couldn't remove attachments, and the reason was that /Trash/TrashAttachment topic didn't exist. It took us a long time to figure this out because the error message we were getting was "You have no access", which led us to investigate user acccess control with no fruit. Eventually, we've realized that all we needed to do was to create /Trash/TrashAttachment. I'd like to propose following
1) Display more accurate error message when this happens - instead of saying "no access", it should say "please create a topic /Trash/TrashAttachment".
2) Also, it will be nice if TWiki actually create Topic if it doesn't exist before trying to remove an attachments.