From: test <test@example.com>
Subject: test multipart mixed, attachment with different charset
To: test2 <test2@example.com>
Date: Sat, 27 Feb 2016 17:11:45 +0100
MIME-Version: 1.0
Content-Type: multipart/mixed;
 boundary="boundary"

This is a multi-part message in MIME format.
--boundary
Content-Type: text/plain; charset=EUC-KR; format=flowed
Content-Transfer-Encoding: 7bit

Hi there.

--boundary
Content-Type: text/plain; charset=KOI8-R;
 name="attachment.tmx"
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment;
 filename="attachment.tmx"

Just some text.
--boundary--