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

This part should not be displayed.
--------------test-boundary
Content-Type: text/plain; name="note.txt"
Content-Disposition: attachment; filename="note.txt"
Content-Transfer-Encoding: base64

QXR0YWNoZWQgcGxhaW4gdGV4dCBwYXlsb2FkLgo=
--------------test-boundary--