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

--longrandomstring

body
--longrandomstring
Content-Type: application/octet-stream; name="attachment-contenttype.txt"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="attachment-disposition.txt"

aWYgeW91IGFyZSByZWFkaW5nIHRoaXMsIGhpIQ==
--longrandomstring--