Subject: see attachment
Message-ID: <file@attached>
From: "John Smith" <js@example.com>
To: list@example.com
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: multipart/mixed; boundary="b"

--b
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment; filename="attached_fart.txt"

inside the attachment=

--b
Content-Type: text/plain; charset="US-ASCII"
Content-Disposition: attachment; filename="part_deux.txt"
Content-Transfer-Encoding: quoted-printable

inside another=

--b--