From: Alice Example <alice@example.com>
To: Bob Example <bob@example.net>
Subject: Multipart probe
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="BST-MIXED-1"

This preamble is not a part.
--BST-MIXED-1
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Part-Index: 1

plain part
--BST-MIXED-1
Content-Type: application/octet-stream; name="payload.bin"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="payload.bin"
X-Part-Index: 2

YnJpbnRPUwAB/3dhc20zMgo=
--BST-MIXED-1
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
X-Part-Index: 3

caf=C3=A9 na=C3=AFve =3D sign
a second line long enough that the quoted-printable encoder must insert a s=
oft line break somewhere in it
tab	here and trailing space=20
--BST-MIXED-1--
This epilogue is not a part either.