From: Sender <sender@pm.me>
To: Receiver <receiver@pm.me>
Content-Type: multipart/mixed; boundary=longrandomstring

--longrandomstring

body
--longrandomstring
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64

aWYgeW91IGFyZSByZWFkaW5nIHRoaXMsIGhpIQ==
--longrandomstring--