Log into the administrative options for your list. Click on content filtering. The very first option you can change is: "Should Mailman filter the content of list traffic according to the settings below?" change this setting to "yes" then submit changes. By default the list will start to remove attachments that are sent to the list. and remove HTML tags leaving just plain text. The options below this allow you to determine what kind of attachments or formatting in messages is or isn't allowed (like allowing .jpg pictures but not word documents). To simply remove attachments though, no other changes should be required.