Add attachment(s) to this email

        :param attachment: Add attachment(s) to this email
        :type attachment: Attachment, list(Attachment)

def attachment(self, attachment):
        """Add attachment(s) to this email

        :param attachment: Add attachment(s) to this email
        :type attachment: Attachment, list(Attachment)
        """
        if isinstance(attachment, list):
            for a in attachment:
                self.add_attachment(a)
        else:
            self.add_attachment(attachment)