Return the message's content type.
The returned string is coerced to lower case of the form
'maintype/subtype'. If there was no Content-Type header in the
message, the default type as given by get_default_type() will be
returned. Since according to RFC 2045, messages always have a default
type this will always return a value.
RFC 2045 defines a message's default type to be text/plain unless it
appears inside a multipart/digest container, in which case it would be
message/rfc822.