MIME Type Structure
Each MIME media type consists of a type, a subtype, and a list of optional parameters. The type and subtype are separated by a slash, and the optional parameters begin with a semicolon, if they are present. In HTTP, MIME media types are widely used in Content-Type and Accept headers. Here are a few examples:
Content-Type: video/quicktime
Content-Type: text/html; charset="iso-8859-6"
Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08j34c0p
Accept: image/gif
D.2.1 Discrete Types
MIME types can directly describe the object type, or they can describe collections or packages of other object types. If a MIME type describes an object type directly, it is a discrete type. These include text files, videos, and application-specific file formats.
D.2.2 Composite Types
If a MIME type describes a collection or encapsulation of other content, the MIME type is called a composite type. A composite type describes the format of the enclosing package. When the enclosing package is opened, each enclosed object will have its own type.
D.2.3 Multipart Types
Multipart media types are composite types. A multipart object consists of multiple component types. Here's an example of multipart/mixed content, where each component has its own MIME type:
Content-Type: multipart/mixed; boundary=unique-boundary-1
--unique-boundary-1
Content-type: text/plain; charset=US-ASCII
Hi there, I'm some boring ASCII text...
--unique-boundary-1
Content-Type: multipart/parallel; boundary=unique-boundary-2
--unique-boundary-2
Content-Type: audio/basic
... 8000 Hz single-channel mu-law-format
audio data goes here ...
--unique-boundary-2
Content-Type: image/jpeg
... image data goes here ...
--unique-boundary-2--
--unique-boundary-1
Content-type: text/enriched
This is <bold><italic>enriched.</italic></bold>
<smaller>as defined in RFC 1896</smaller>
Isn't it <bigger><bigger>cool?</bigger></bigger>
--unique-boundary-1
Content-Type: message/rfc822
From: (mailbox in US-ASCII)
To: (address in US-ASCII)
Subject: (subject in US-ASCII)
Content-Type: Text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: Quoted-printable
... Additional text in ISO-8859-1 goes here ...
--unique-boundary-1--
D.2.4 Syntax
As we stated earlier, MIME types consist of a primary type, a subtype, and an optional list of parameters.
The primary type can be a predefined type, an IETF-defined extension token, or an experimental token (beginning with "x-"). Some common primary types are described in Table D-1.
Table D-1. Common primary MIME types | |
| Type | Description |
| application | Application-specific content format (discrete type) |
| audio | Audio format (discrete type) |
| chemical | Chemical data set (discrete IETF extension type) |
| image | Image format (discrete type) |
| message | Message format (composite type) |
| model | 3-D model format (discrete IETF extension type) |
| multipart | Collection of multiple objects (composite type) |
| text | Text format (discrete type) |
| video | Video movie format (discrete type) |
Subtypes can be primary types (as in "text/text"), IANA-registered subtypes, or experimental extension tokens (beginning with "x-").
Types and subtypes are made up of a subset of US-ASCII characters. Spaces and certain reserved grouping and punctuation characters, called "tspecials," are control characters and are forbidden from type and subtype names.
The grammar from RFC 2046 is shown below:
TYPE := "application" | "audio" | "image" | "message" | "multipart" |
"text" | "video" | IETF-TOKEN | X-TOKEN
SUBTYPE := IANA-SUBTOKEN | IETF-TOKEN | X-TOKEN
IETF-TOKEN := <extension token with RFC and registered with IANA>
IANA-SUBTOKEN := <extension token registered with IANA>
X-TOKEN := <"X-" or "x-" prefix, followed by any token>
PARAMETER := TOKEN "=" VALUE
VALUE := TOKEN / QUOTED-STRING
TOKEN := 1*<any (US-ASCII) CHAR except SPACE, CTLs, or TSPECIALS>
TSPECIALS := "(" | ")" | "<" | ">" | "@" |
"," | ";" | ":" | "\" | <"> |
"/" | "[" | "]" | "?" | "="