No, admins don't have access directly, unless they go straight to the database instead of using the actual forums frontend, which doesn't count - not something they do except under extreme circumstances anyway, PM related or not. Other forums have an admin PM hack making it easy for admins to snoop through PMs of anyone they want, but it's not installed at this place. Attachments would be more of a concern than normal spammy PMs, because they take up a heck of a lot more space.
