Message-ID: <attachment-in-multipart-alternative@example.invalid>
Subject: Attachment inside multipart/alternative
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="------alternative"

--------alternative
Content-Type: text/plain
Content-Transfer-Encoding: 7bit

This is the plain text body.

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

QXR0YWNobWVudCBpbiBtdWx0aXBhcnQvYWx0ZXJuYXRpdmUu

--------alternative--