MIME Type Structure

D.2 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 := "("  ")"  "<"  ">"  "@" 

 ","  ";"  ":"  "\"  <"> 

 "/"  "["  "]"  "?"  "="