Media type

In information and communications technology, a media type,^[1][2] content type^[2][3] or MIME type^[1][4][5] is a two-part identifier for file formats and content formats. Their purpose is comparable to filename extensions and uniform type identifiers, in that they identify the intended data format. They are mainly used by technologies underpinning the Internet, and also used on Linux desktop systems.

This article may be confusing or unclear to readers. (April 2023)

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 RFC 2045 (MIME) Part One: Format of Internet Message Bodies (Nov 1996) in November 1996 as a part of the MIME (Multipurpose Internet Mail Extensions) specification, for denoting type of email message content and attachments;^[6] hence the original name, MIME type. Media types are also used by other internet protocols such as HTTP,^[7] document file formats such as HTML,^[8] and the XDG specifications implemented by Linux desktop environments,^[5] for similar purposes.

Different internet standards or web standards bodies differ on the preferred term for this type of identifier.

The IANA and IETF use the term "media type", and consider the term "MIME type" to be obsolete,^[1] since media types have become used in contexts unrelated to email, such as HTTP. By contrast, the WHATWG continues to use the term "MIME type" and discourages use of the term "media type" as ambiguous, since it is used with a different meaning in connection with the CSS @media feature.^[4]

The HTTP response header for providing the media type is Content-Type.^[2] The W3C has used ContentType as an XML data-type name for a media type.^[3] XDG specifications implemented by Linux desktop environments continue to use the term "MIME type".^[5]

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:

mime-type = type "/" [tree "."] subtype ["+" suffix]* [";" parameter];

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.

Types, subtypes, and parameter names are case-insensitive. Parameter values are usually case-sensitive, but may be interpreted in a case-insensitive fashion depending on the intended use.^[6]

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 RFC 1524 "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.

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 application/octet-stream, and 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 /etc/mime.types and/or $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 .htm and .html by the following line:

text/html     htm   html

Netscape use

This section is missing information about Netscape. (November 2019)

The mime.types file dates to Netscape, where it used a different format;^[20] 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

• Content negotiation
• Content sniffing
• Filename extension