Content-type is a email headers that defines:
Where the first parameter is the media type (mime)
Content-Type: text/html; charset="UTF-8"
Content-Type: text/plain; charset="UTF-8"
Content-Type: message/rfc822
Content-Type: message/delivery-status
Content-Type: image/png; name="icon.png"
Content-Disposition: attachment; filename="icon.png"
Content-Transfer-Encoding: base64
Content-ID: <icon.png>
Multipart media type means that the body is composed of multiple single part
Content-Type: multipart/alternative; boundary="0000000000008c12850607c40b30"
Content-Type: multipart/related; boundary="0000000000008c12780607c40b2f"
Content-Type: multipart/report; boundary="0000000000008c07b00607c40b27"; report-type=delivery-status
It has the same functionality and syntax than the HTTP content type header
Namely:
Content-Type: mediaType; parameter1=value1; parameter2=value2
where: