Set the mimetype to the value.

def mimetype(self, value: str) -> None:
        """Set the mimetype to the value."""
        if (
                value.startswith('text/') or value == 'application/xml' or
                (value.startswith('application/') and value.endswith('+xml'))
        ):
            mimetype = f"{value}; charset={self.charset}"
        else:
            mimetype = value
        self.headers['Content-Type'] = mimetype