Most of my activesync attachments are fine but the ones from my wife's blackberry are garbled (the name, and also content when downloaded).  They are fine if I use IMAP or the web client.  I have also seen unreadable HTML messages from one other person, again fine when not using activesync.  I have ATT Tilt with WM 6.1.  My guess is that there is some sort of encoding incompatibility when using activesync.