See:

http://www.codefab.com/mailman-dav/

• decodes MIME attachments in incoming email

• archive attachments to WebDAV enabled server

• rewrite email messages to contain URL to attachments, instead of actual
  attachments [URL can be different than DAV URL]

• send message to HTTP server [can be used to notify a web based system of
  incoming email messages]

• fully configurable via web based GUI