Verify that attachment values match the format expected by the
        Postmark API.

def _verify_attachments(self):
        '''Verify that attachment values match the format expected by the
        Postmark API.
        '''
        if self.attachments is None:
            return
        keys = ('Name', 'Content', 'ContentType')
        self._verify_dict_list(self.attachments, keys, 'Attachment')