From: sender@example.invalid
To: recipient@example.invalid
Subject: Synthetic size example
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="engine1-example"

--engine1-example
Content-Type: text/plain; charset=utf-8

Synthetic fixture, not a real message.
--engine1-example
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="sample.bin"

QUJD
--engine1-example--