Block attachments by default

[Larsnooden-users]

Netiquette requires that people refrain from sending attachments to
lists. Most mailing lists have have policy anyway to explicity
request that user respect eachother's bandwidth. Thus for these
groups any mail that arrives with an attachment is probably an mS-
virus or mS-worm and not welcome anyway. So Mailman ought to
have a function to block messages with attachments - and that
function ought to be turned on by default. There are exceptions
though, so it should be possible to turn off this blocking for individual
lists.

There could be several behaviors to do this:

1) send the whole thing to /dev/null

2) send the attachment to /dev/null and let the text-based body
through

3) do either of the above and send a warning to the sender
explaining what happened

etc.

[http://sourceforge.net/tracker/index.php?func=detail&aid=1204262&group_id=103&atid=100103]