MIME types demystified

Most programmers know what a MIME type (or Media Type) is. It's the two-part identifier used by programs to determine the format of a file or data. In web development, MIME types are most commonly seen in HTTP requests as the value of the Content-Type header. But what do the two different parts mean and what are their valid values?

The first part of a MIME type is called the top-level media type and it specifies the general type of the data. The top-level media type can be one of the 7 standard defined types, which consist of:

• text - textual information.
• image - image data.
• audio - audio data.
• video - video data.
• application - some other kind of data, typically either uninterpreted binary data or information to be processed by an application.
• multipart - data consisting of multiple entities of independent data types.
• message - an encapsulated message.

The second part is called the sub-type and it specifies the specific format of the data. There are thousands of registered sub-types, far too many to list here. However there's a list of registered MIME types on the IANA website.

The sub-type can also have one of the following optional prefixes to give it special meaning:

• x- - Unregistered or experimental. Example: image/x-icon
• vnd. - Vendor-specific. Example: application/vnd.ms-powerpoint
• prs. - Personal or vanity. Example: application/prs.roland

For more information, see the MIME type standard and registration documents.