"Content-Disposition is an optional header field. In its absence, the MUA may use whatever presentation method it deems suitable." — RFC 2183

The email.message.Message class should gain a get_content_disposition() method with the three possible return values 'inline', 'attachment', and None so that email clients can easily distinguish between the three states described in the RFC.

See also the discussion at http://bugs.python.org/issue21079