From: a@example.com
Subject: blah
Content-Type: multipart/mixed; boundary="b"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

--b
Content-Type: application/vnd.ms-excel
Content-Transfer-Encoding: base64

anVuaw==

--b
Content-Type: text/plain
Content-Transfer-Encoding: quoted-printable

junk=

--b--