Internet Media Type

An Internet media type is a two-part identifier for file formats on the Internet. The identifiers were originally defined in RFC 2046 for use in email sent through SMTP, but their use has expanded to other protocols such as HTTP, RTP and SIP. These types were called MIME types, and are sometimes referred to as Content-types, after the name of a header in several protocols whose value is such a type. The original name MIME type referred to usage to identify non-ASCII parts of email messages composed using the MIME (Multipurpose Internet Mail Extensions) specification. Without MIME types, email clients would not be able to understand if an attachment file were a graphics file or a spreadsheet etc. and would not be able to handle the attachment appropriately.

A media type is composed of two or more parts: A type, a subtype, and zero or more optional parameters. For example, subtypes of text have an optional charset parameter that can be included to indicate the character encoding (e.g. text/html; charset=UTF-8), and subtypes of multipart type often define a boundary between parts. Allowed charset values are defined in the list of IANA character sets.

Prior to RFC 6648, experimental or non-standard media types were prefixed with x-, but this practice was deprecated due to incompatibility problems when the experimental types were standardized. Subtypes that begin with vnd. are vendor-specific; subtypes in the personal or vanity tree begin with prs.. New media types can be created with the procedures outlined in RFC 4288.

In addition to email clients, web browsers also support various media types. This enables the browser to display or output files that are not in HTML format. Media type specification is also an important information source for search engines for the classification of data files on the web.

There are many registered media types, such as GIF graphics files and PostScript files. It is also possible to define custom media types.