Why are footers being shown as attachments to HTML-formatted messages?
Mailman adds the footer as an attachment if the original message posted contains a message formatted in HTML MIME, or a text/plain MIME body part using a different character set than what Mailman would use for the footers.
If you do not like this behavior, one easy workaround is to completely disable the footer that is added. Go to your Mailman list administration page.
Under Non-digest options
Clear all the text in this input box, and be sure not to leave any blank lines, to disable adding the message footer for regular list members.
Under Digest options
Clear all the text in this input box, and be sure not to leave any blank lines, to disable adding the message footer for digest list members.