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

--longrandomstring

body
--longrandomstring
Content-Type: text/calendar; charset=utf-8; method=REQUEST; name=invite.ics'
Content-Transfer-Encoding: Base64
Content-Disposition: attachment; filename=invite.ics

VGhpcyBpcyBhbiBpY3MgY2FsZW5kYXIgaW52aXRl
--longrandomstring--