'The extension data of a non-multipart body part are in the
following order:
1. body MD5
A string giving the body MD5 value as defined in [MD5].
2. body disposition
A parenthesized list with the same content and function as
the body disposition for a multipart body part.
3. body language
A string or parenthesized list giving the body language
value as defined in [LANGUAGE-TAGS].
4. body location
A string list giving the body content URI as defined in
[LOCATION].'

def _extended(self):

    result = []
    headers = self.message.getHeaders(False, 'content-md5', 'content-disposition', 'content-language', 'content-language')
    result.append(headers.get('content-md5'))
    result.append(self._disposition(headers.get('content-disposition')))
    result.append(headers.get('content-language'))
    result.append(headers.get('content-location'))
    return result
