'Returns a new instance of MimeWriter which represents an
individual part in a multipart message.
This may be used to write the part as well as used for creating
recursively complex multipart messages. The message must first be
initialized with the startmultipartbody() method before using the
nextpart() method.'

def nextpart(self):

    self._fp.write((('\n--' + self._boundary) + '\n'))
    return self.__class__(self._fp)
