By default, a new "group discussion" Mailman list does not allow list messages with attachments.
One reason for this is that allowing attachments can theoretically allow harmful things like viruses to be posted (although our virus scanner will catch almost all of those). The other reason most mailing lists don't allow attachments is that many e-mail programs send multiple versions of the message in different formats as multiple attachments, which may be annoying for list members.
If you wish, you can allow attachments to be sent to your Mailman group discussion mailing list. To do so, just turn off "content filtering" in Mailman.
Go to the management page for your list, then click Content filtering.
Click No next to "Should Mailman filter the content of list traffic according to the settings below?"
Your Mailman list will now allow attachments to be sent. (Note that this change also allows HTML messages and other non-plain-text messages.)