4.13. How do I prevent MIME attachments/HTML/Viruses being sent to lists
See also the Mailman FAQ entries at How do I turn off HTML messages-attachments? and 4.39 Mailman is munging HTML & MIME-formatted messages before they are sent out? (problems with Mailman 2.1.x footers).
From posting by Alf Christophersen, to mailman-developers on 7 Dec 01
Most of these viruses/attachments come in as MIME encoded, so you can always use stripmime, or demime to get that stuff off of the message prior to submitting it to the list. You can go one step further, and look into MIMEDefang, which does a plethora of things for you. There's also Quarantine, which will strip out any type of attachments to your message. All of these solutions are external to Mailman (in fact MIMEDefang hooks into sendmail - if you're running it), so you can always end up with "clean" messages going through your list.
An alternative is a MIME handler patch for mailman itself - see
Converted from the Mailman FAQ Wizard
This is one of many Frequently Asked Questions.