Learning XML, 2nd Edition

MIME and Media Types

The Multipurpose Internet Mail Extensions (MIME) standard is a means of specifying media types such as images, program data , audio files, and text. Described in Internet Engineering Task Force (IETF) Request for Comments (RFC) documents 2045 through 2049, it includes a comprehensive list of known types and has inspired a registry for many more.

MIME was developed originally to extend the paradigm of email from plain text to a rich array of media. Email transport systems, such as the Simple Mail Transfer Protocol (SMTP), can only deal with 7-bit ASCII text. You cannot simply append a binary file to the end of a message and have it bounce happily across the Internet. The data has to be encoded in an ASCII-compatible way. There are other requirements as well, such as a minimum line length and absence of certain control characters. MIME introduces methods to transform data into a safe form. It also describes how to package this data in a recognizable way for mail transfer agents and clients to work with.

One of the ways MIME describes a resource is by assigning it a media type (or content-type) which names the general category that best describes the data. Each type includes a set of subtypes that exactly identify the resource. The type and subtype are usually written together, joined by a slash character (/). For example, image/jpeg denotes a graphical resource in the JPEG format. The major types include:

text

Textual information that can be read in a traditional ...