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A media type (formerly known as MIME type) is a two-part identifier for file formats and format contents transmitted on the Internet. The Internet Assigned Numbers Authority (IANA) is the official authority for the standardization and publication of these classifications. Media types were originally defined in Request for Comments 2045 in November 1996 as a part of MIME (Multipurpose Internet Mail Extensions) specification, for denoting type of email message content and attachments; hence the name MIME type. Media types are also used by other internet protocols such as HTTP and document file formats such as HTML, for similar purpose.
A media type consists of a type and a subtype, which is further structured into a tree. A media type can optionally define a suffix and parameters:
type "/" [tree "."] subtype ["+" suffix] *[";" parameter]
The currently registered types are:
As an example, an HTML file might be designated
text/html; charset=UTF-8. In this example,
text is the type,
html is the subtype, and
charset=UTF-8 is an optional parameter indicating the character encoding.
A subtype typically consists of a media format, but it may or must also contain other content, such as a tree prefix, producer, product or suffix, according to the different rules in registration trees.
All media types should be registered using the IANA registration procedures. For the efficiency and flexibility of the media type registration process, different structures of subtypes can be registered in registration trees that are distinguished by the use of tree prefixes. Currently the following trees are created: standard (no prefix), vendor (
vnd. prefix), personal or vanity (
prs. prefix), unregistered (
x. prefix). These registration trees were first defined in November 1996 (obsoleted RFC 2048 - currently RFC 6838). New registration trees may be created by IETF Standards Action for external registration and management by well-known permanent organizations (e.g. scientific societies).
The standards tree does not use any tree prefix:
type "/" subtype ["+" suffix] *[";" parameter]
Registrations in the standards tree must be either associated with IETF specifications approved directly by the IESG, or registered by an IANA recognized standards-related organization.
The vendor tree includes media types associated with publicly available products. It uses the
vnd. tree prefix:
type "/" "vnd." subtype ["+" suffix] *[";" parameter]
The terms "vendor" and "producer" are considered equivalent in the context. Industry consortia as well as non-commercial entities can register media types in the vendor tree. A registration in the vendor tree may be created by anyone who needs to interchange files associated with some software product or set of products. However, the registration belongs to the vendor or organization producing the software that employs the type being registered, and that vendor or organization can at any time elect to assert ownership of a registration done by a third party.
The personal or vanity tree includes media types associated with non publicly available products or experimental media types. It uses the
prs. tree prefix:
type "/" "prs." subtype ["+" suffix] *[";" parameter]
The unregistered tree includes media types intended exclusively for use in private environments and only with the active agreement of the parties exchanging them. It uses the
x. tree prefix:
type "/" "x." subtype ["+" suffix] *[";" parameter]
Media types in this tree cannot be registered. According to RFC 6838 (published in January 2013), any use of types in the unregistered tree is strongly discouraged. In addition, subtypes prefixed with
X- are no longer considered to be members of this tree.
According to obsoleted RFC 2048 (published in November 1996)--it should rarely, if ever, be necessary to use unregistered types, and as such use of any
X- prefixes is discouraged. Obsoleted RFC 1590 (published in September 1993) stated that the use of the
X- prefix may be used for unregistered subtypes.
Media types that have been widely deployed (with a subtype prefixed with
X-) without being registered, should be, if possible, re-registered with a proper prefixed subtype. If this is not possible, the media type can, after an approval by both the media types reviewer and the IESG, be registered in the standards tree with its unprefixed subtype.
Suffix is an augmentation to the media type definition to additionally specify the underlying structure of that media type. Media types that make use of a named structured syntax should use the appropriate IANA registered
"+"suffix for that structured syntax when they are registered. Unregistered suffixes should not be used (since January 2013). Structured syntax suffix registration procedures are defined in RFC 6838.
Mailcap (derived from the phrase "mail capability") is a type of meta file used to configure how MIME-aware applications such as mail clients and web browsers render files of different MIME-types. The mailcap format is defined by RFC1524 "A User Agent Configuration Mechanism for Multimedia Mail Format Information) but is not defined as an Internet standard. It is supported by most Unix systems.
Lines can be comments starting with the # character, or a mime-type followed by how to handle that mime type. The first part is called the content-type, and the second part is called the view-command. For example,
video/mpeg; xmpeg %s says if a file encoded in mime has type
video/mpeg, run the xmpeg program with the file name as a parameter.
An associated file is the mime.types file, which associates filename extensions with a MIME type. If the MIME type is properly set, this is unnecessary, but MIME types may be incorrectly set, or set to a generic type such as
mime.types allows one to fall back on the extension in these cases. Similarly, since many file systems do not store MIME type information, but instead rely on the filename extension, a mime.types file is frequently used by web servers to determine MIME type.
When viewing a file, these two work together as follows:
mime.types associates an extension with a MIME type, while
mailcap associates a MIME type with a program.
In UNIX-type systems, the mime.types file is usually located at
$HOME/.mime.types and the format is simply that each line is a space-delimited list of a MIME type, followed by zero or more extensions. For example, the HTML type can be associated with the extensions
.html by the following line:
text/html htm html
The mime.types file dates to Netscape, where it used a different format; it used key-value pairs and a comma-separated list of extensions, together with a standard header consisting of a specific comment that identifies the file as a mime.types file, as follows.
#--Netscape Communications Corporation MIME Information # Do not delete the above line. It is used to identify the file type. type=text/html exts=htm,html
This article is missing information about the Shared MIME-info standard. (August 2016)
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