Skip to main content

5. Composite Media Type Values

The two composite top-level media types are: multipart and message.

5.1. Multipart Media Type​

The multipart type is used to combine multiple body parts into a single message. Each part is an independent entity with its own headers and body.

5.1.1. Common Syntax​

All multipart types share a common syntax. The boundary parameter is required:

Content-Type: multipart/mixed; boundary="----=_Part_0_123456789.987654321"

------=_Part_0_123456789.987654321
Content-Type: text/plain

First part content

------=_Part_0_123456789.987654321
Content-Type: text/html

<html>Second part content`</html>`

------=_Part_0_123456789.987654321--

5.1.3. Mixed Subtype​

multipart/mixed is used for independent parts that need to be presented in order.

Use Case: Email body + attachments

Content-Type: multipart/mixed; boundary="boundary123"

--boundary123
Content-Type: text/plain

Email body

--boundary123
Content-Type: application/pdf
Content-Disposition: attachment; filename="document.pdf"

[PDF data]

--boundary123--

5.1.4. Alternative Subtype​

multipart/alternative is used for different representations of the same content. Mail clients should select the last version they understand.

Use Case: Plain text + HTML versions

Content-Type: multipart/alternative; boundary="alt123"

--alt123
Content-Type: text/plain

Plain text version

--alt123
Content-Type: text/html

<html><body>HTML version`</body>``</html>`

--alt123--

5.1.5. Digest Subtype​

multipart/digest is used for forwarding multiple messages. The default part type is message/rfc822.

5.1.6. Parallel Subtype​

multipart/parallel indicates that parts should be presented simultaneously.

5.1.7. Other Multipart Subtypes​

  • multipart/related: Related parts (e.g., HTML + embedded images)
  • multipart/form-data: HTML form file uploads
  • multipart/byteranges: HTTP range requests

5.2. Message Media Type​

The message type is used to encapsulate another message.

5.2.1. RFC822 Subtype​

message/rfc822 is used to encapsulate a complete RFC 822 message.

Content-Type: message/rfc822

From: [email protected]
To: [email protected]
Subject: Forwarded message

This is the forwarded message content.

5.2.2. Partial Subtype​

message/partial is used to split large messages into multiple fragments.

Parameters:

  • id: Unique identifier
  • number: Fragment number
  • total: Total number of fragments
Content-Type: message/partial; 
    id="[email protected]";
    number=1;
    total=3

5.2.3. External-Body Subtype​

message/external-body is used to reference data stored elsewhere.

Access Types:

  • ftp: FTP server
  • http: HTTP URL
  • local-file: Local file
  • mail-server: Mail server
Content-Type: message/external-body;
    access-type=URL;
    URL="http://example.com/file.pdf"

Content-Type: application/pdf
Content-Description: Large PDF File

Typical Nested Structure:

multipart/mixed
├── text/plain (body)
├── multipart/alternative
│   ├── text/plain
│   └── text/html
└── application/pdf (attachment)
Last updated on