To: to@here.com
From: from@there.com
Subject: Plain text attachment
Date: Tue, 21 Aug 2018 09:05:14 +0200
MIME-Version: 1.0
Content-Type: multipart/mixed;
 boundary="------------B832AF745285AEEC6D5AEE42"

This is a multi-part message in MIME format.
--------------B832AF745285AEEC6D5AEE42
Content-Type: text/plain; charset=iso-8859-15; format=flowed
Content-Transfer-Encoding: 7bit

Test
--------------B832AF745285AEEC6D5AEE42
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename="a.txt"

SGkh
--------------B832AF745285AEEC6D5AEE42--