Does the filename.rules.rules file also contain the same line? Do the files they refer to exist? (i.e. /usr/mailscanner/etc/filetype.no.rules.conf). What exactly is the message you are getting in MailWatch, or the report received by the sender of the attachment?
Not sure what could be causing this. We have seen at least one case recently where it was a specific email or a specific attachment that had this problem, but other emails with attachment could successfully be released from quarantine. We never were able to determine why the original email could not be released.