I note that many have the almost identical problem in the "unsupported" hack version for 3.5 LOL
https://vborg.vbsupport.ru/showthread.php?t=91220 but unfortunately the author does not appear to have resolved the issue there either.
Its a real shame if it cannot be corrected another add on that I would have like to have seen would be to specify user groups which can or cannot use PM attachments set in admin cp.
If someone can resolve the issue I would be willing to make a donation