3.4 MIME Media Types

     

MIME is an open standard for sending multipart, multimedia data through Internet email. The data may be binary, or it may use multiple ASCII and non-ASCII character sets. Although MIME was originally intended just for email, it has become a widely used technique to describe a file's contents so that client software can tell the difference between different kinds of data. For example, a web browser uses MIME to tell whether a file is a GIF image or a printable PostScript file.

Officially, MIME stands for Multipurpose Internet Mail Extensions, which is the expansion of the acronym used in RFC 2045. However, you will hear other versionsmost frequently Multipart Internet Mail Extensions and Multimedia Internet Mail Extensions.

MIME supports more than 100 predefined types of content. Content types are classified at two levels: a type and a subtype. The type shows very generally what kind of data is contained: is it a picture, text, or movie? The subtype identifies the specific type of data: GIF image, JPEG image, TIFF image. For example, HTML's content type is text/html ; the type is text , and the subtype is html . The content type for a GIF image is image/gif ; the type is image , and the subtype is gif . Table 3-2 lists the more common defined content types. On most systems, a simple text file maintains a mapping between MIME types and the application used to process that type of data; on Unix, this file is called mime.types . The most current list of registered MIME types is available from http://www.iana.org/assignments/media-types/. For more on MIME, see the comp.mail.mime FAQ at http://www.uni-giessen.de/faq/archiv/mail.mime-faq.part1-9/.

Web servers use MIME to identify the kind of data they're sending. Web clients use MIME to identify the kind of data they're willing to accept. Most web servers and clients understand at least two MIME text content types, text/html and text/plain , and two image formats, image/gif and image/jpeg . More recent browsers also understand application/xml and several other image formats. Java relies on MIME types to pick the appropriate content handler for a particular stream of data.

Table 3-2. Predefined MIME content types

Type	Subtype	Description
text		The document represents printable text.
	calendar	Calendaring and scheduling information in the iCalendar format; see RFC 2445.
	css	A Cascading Style Sheet used for HTML and XML.
	directory	Address book information such as name , phone number, and email address; used by Netscape vCards; defined in RFCs 2425 and 2426.
	enriched	A very simple HTML-like language for adding basic font and paragraph-level formatting such as bold and italic to email; used by Eudora; defined in RFC 1896.
	html	Hypertext Markup Language as used by web browsers.
	plain	This is supposed to imply raw ASCII text. However, some web servers use text/plain as the default MIME type for any file they can't recognize. Therefore, anything and everything, most notably .class byte code files, can get identified as a text/plain file.
	richtext	An HTML-like markup for encoding formatting into pure ASCII text. It's never really caught on, in large part because of the popularity of HTML.
	rtf	An incompletely defined Microsoft format for word processing files.
	sgml	The Standard Generalized Markup Language; ISO standard 8879:1986.
	tab-separated-values	The interchange format used by many spreadsheets and databases; records are separated by linebreaks and fields by tabs.
	xml	The W3C standard Extensible Markup Language. For various technical reasons, application/xml should be used instead, but often isn't.
multipart		Multipart MIME messages encode several different files into one message.
	mixed	Several message parts intended for sequential viewing.
	alternative	The same message in multiple formats so a client may choose the most convenient one.
	digest	A popular format for merging many email messages into a single digest; used by many mailing lists and some FAQ lists.
	parallel	Several parts intended for simultaneous viewing.
	byteranges	Several separately contiguous byte ranges; used in HTTP 1.1.
	encrypted	One part for the body of the message and one part for the information necessary to decode the message.
	signed	One part for the body of the message and one part for the digital signature.
	related	Compound documents formed by aggregating several smaller parts.
	form-data	Form responses.
message		An email message.
	external-body	Just the headers of the email message; the message's body is not included but exists at some other location and is referenced, perhaps by a URL.
	http	An HTTP 1.1 request from a web client to a web server.
	news	A news article.
	partial	Part of a longer email message that has been split into multiple parts to allow transmission through email gateways.
	rfc822	A standard email message including headers.
image		Two-dimensional pictures.
	cgm	A Computer Graphics Metafile format image. CGM is ISO standard 8632:1992 for device-independent vector graphics and bitmap images.
	g3fax	The standard for bitmapped fax images.
	gif	A Graphics Interchange Format image.
	jpeg	The Joint Photographic Experts Group file format for bitmapped images with lossy compression.
	png	A Portable Network Graphics Format image. The format was developed at the W3C as a modern replacement for GIF that supports 24-bit color and is not encumbered by patents.
	tiff	The Tagged Image File format from Adobe.
audio		Sound.
	basic	8-bit ISDN -law encoded audio with a single channel and a sample rate of eight kilohertz. This is the format used by .au and .snd files and supported by the java.applet.AudioClip class.
video		Video.
	mpeg	The Motion Picture Experts Group format for video data with lossy compression.
	quicktime	Apple's proprietary QuickTime movie format. Before being included in a MIME message, QuickTime files must be "flattened".
model		3-D images.
	vrml	A Virtual Reality Modeling Language file, a format for 3-D data on the Web.
	iges	The Initial Graphics Exchange Specification for interchanging documents between different CAD programs.
	mesh	The mesh structures used in finite element and finite difference methods .
application		Binary data specific to some application.
	octet-stream	Unspecified binary data, which is usually saved into a file for the user . This MIME type is sometimes used to serve .class byte code files.
	java	A nonstandard subtype sometimes used to serve .class byte code files.
	postscript	Adobe PostScript.
	dca-rft	IBM's Document Content Architecture-Richly Formatted Text.
	mac-BinHex40	A means of encoding the two forks of a Macintosh document in a single ASCII file.
	pdf	An Adobe Acrobat file.
	zip	A zip compressed file.
	macwriteii	A MacWrite II word-processing document.
	msword	A Microsoft Word document.
	xml+xhtml	An XHTML document
	xml	An Extensible Markup Language document.

A MIME-compliant program is not required to understand all these different types of data; it just needs to recognize what it can and cannot handle. Many programsNetscape Navigator, for exampleuse various helper programs to display types of content they themselves don't understand.

MIME allows you to define additional nonstandard subtypes by using the prefix x- . For example, the content type application/x-tex has the MIME type application and the nonstandard subtype x-tex for a TeX document. These x-types are not guaranteed to be understood by any program other than the one that created them. Indeed, two programs may use the same x-type to mean two completely different things, or different programs may use different x-types to mean the same thing. However, many nonstandard types have come into common use; some of the more common ones are listed in Table 3-3.

Table 3-3. X-types

Type	X-subtype	Description
application		Subtypes of an application; the name of the subtype is usually a file format name or an application name.
	x-aiff	SGI's AIFF audio data format.
	x-bitmap	An X Windows bitmap image.
	x-gzip	Data compressed in the GNU gzip format.
	x-dvi	A TeX DVI document.
	x-framemaker	A FrameMaker document.
	x-latex	A LaTeX document.
	x-macBinHex40	Identical to application/mac-BinHex40 , but older software may use this x-type instead.
	x-mif	A FrameMaker MIF document.
	x-sd	A session directory protocol announcement, used to announce MBONE events.
	x-shar	A shell archive; the Unix equivalent of a Windows or Macintosh self-extracting archive. Software shouldn't be configured to unpack shell archives automatically, because a shell archive can call any program the user who runs it has the rights to call.
	x-tar	A tar archive.
	x-gtar	A GNU tar archive.
	x-tcl	A tool command language (TCL) program. You should never configure your web browser or email program to automatically run programs you download from the web or receive in email messages.
	x-tex	A TeX document.
	x-texinfo	A GNU texinfo document.
	x-troff	A troff document.
	x-troff-man	A troff document written with the man macros.
	x-troff-me	A troff document that should be processed using the me macros.
	x-troff-ms	A troff document that should be processed using the ms macros.
	x-wais-source	A WAIS source.
	x-www-form-urlencoded	A string that has been encoded like a URL, with + replacing spaces and % escapes replacing non- alphanumeric characters that aren't separators.
audio
	x-aiff	The same as application/x-aiff : an AIFF audio file.
	x-mpeg	The MP3 sound format.
	x-mpeg.mp3	The MP3 sound format.
	x-wav	The Windows WAV sound format.
image
	x-fits	The FITS image format used primarily by astronomers.
	x-macpict	A Macintosh PICT image.
	x-pict	A Macintosh PICT image.
	x-macpaint	A MacPaint image.
	x-pbm	A portable bitmap image.
	x-portable-bitmap	A portable bitmap image.
	x-pgm	A PGM image.
video
	x-msvideo	A Microsoft AVI Video for Windows.
	x-sgi-movie	A Silicon Graphics movie.