The only way to delete files from the File Bank, is to access its listing section (it's hidden on miniBB forums for guests, but you may access it
here) and delete or verify (look up) all files from there. This may help you as a sort of cleaning tool. In theory, it possible to program an automated tool which removes files not used in messages. But the current version has only manual handling of it. If you would like to order a customized script, you are welcome to sponsor it. It's not the same script as in File Attachments. Messages and files do not have associated IDs to synchronize them. That's why the script should go through
all files, through
all messages and some way look them up - how much time it would take on bigger forums?.. This tool should be programmed some tricky way, it's not cheap to develop.