6.2 doexpire.php and doexpire.pl both contain code to delete the attachments. I don't recall if it's actually been tested, but that would be easy enough to do with a test forum.
It's possible that earlier versions didn't delete the attachments.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.