- Choose All on the conditions line.
- Change the first condition line to: Any Attachment Name - Contains - then type a period in the last box.
- Click the + button to add a second condition, and set it to Sender is not in my Address Book.
- Set the Action for this new rule to Move message to mailbox Junk.
[robg adds: This should work well for most people, who tend not to receive attachments from non-regular correspondents. If, like me, you do receive such attachments, you might wish to modify the 'attachment contains' portion of the rule -- specify .PIF, for instance, instead of just a period.]

