Here's one solution, add a filter that matches "Any Header" which is Like 'Content-Type:*boundary*'. This will match any email with boundary's in it, which means the email has multiple parts to it. The downside to this is that an email with both html and text parts will match this rule (which technically is an email with an attachment).

I'd recommend you try out the SpamAssassin plugin, as that should help reduce the amount of spam that reaches your inbox. You can grab it through the admin plugin page.

Adrian