"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.
The docs should say that the value of header is returned, and should also mentioned that the value is lower cased. You should also add a 'versionadded' directive, and for bonus points an entry in the whatsnew document for 3.5. Also, all three possible values should be tested, and a mixed case version of at last one of them, as well as a header that mime parameters in addition to the header value.