3.10. How do I enforce a text/plain posts only policy?

For Mailman 2.0:

In Privacy Options -> Spam-specific posting filters

Hold posts with header value matching a specified regexp.

    Content-Type: .*octet
    Content-Type: .*oda
    Content-Type: .*audio
    Content-Type: .*image
    Content-Type: .*alternative
    Content-Type: .*digest
    Content-Type: .*mixed
    Content-Type: .*related
    Content-Type: .*rich
    Content-Type: .*html
    Content-Type: .*video

Sample Rejection Message:

  Content-Type: (from message header)
  X-Mailer: (from message header)

  Please configure your email client [1] to send text/plain messages to
  this list. Instead of attachments, you should include any diagnostic
  information as in-line text in the main message body.

  [1] http://www.expita.com/nomime.html

  Details:
  Only messages with a Content-type: of "text/plain" and
  "multipart/signed" are automatically posted to the list. All other
  content-types are held for administrative action.

  If you have any questions about this policy, please send them to
  <yourlist-admin@yourdomain>.

An alternate solution is to use a mime stripping script. FAQ 1.8 explains how to do this.

For Mailman 2.1

Use the following Content filtering settings:

filter_content - Yes
filter_mime_types - empty
pass_mime_types

    multipart
    message/rfc822 (if you want to allow plain text parts in attached messages)
    text/plain
    text/html (if you want to allow HTML only posts and convert them to plain text)

filter_filename_extensions - default settings or empty
pass_filename_extensions) - empty
collapse_alternatives - Yes
convert_html_to_plaintext - Yes
filter_action - as desired

This is one of many Frequently Asked Questions.