1.10 Media Types

One of the most important functions of headers is to make it possible for the recipient of the data to know what kind of data it is receiving, and thus be able to process it appropriately. If the client didn't know that the data returned by the server was a GIF image, it wouldn't know how to render it on the screen. If it didn't know that some other data was an audio snippet, it wouldn't know to call up an external helper application. For negotiating different data types, HTTP incorporated Internet Media Types, which look a lot like MIME types but are not exactly MIME types.

The client tells the server which media types it can handle, using the Accept header. The server tries to return information in one of the client's preferred media types, and declares the type of the data using the Content-type header.

The Accept header is used to specify the client's preference for media formats, or to tell the server that it can accept unusual document types. If this header is omitted, the server assumes that the client can accept any media type. The Accept header can have three general forms:

Accept: */*
Accept: type/*
Accept: type/subtype

Using the first form, */*, indicates that the client can accept an entity-body of any media type. The second form, type/*, communicates that an entity-body of a certain general class is acceptable. A client may issue an Accept: image/* to accept images, where the type of image (GIF, JPEG, or whatever) is not important. The third form indicates that an entity-body from a certain type and subtype is acceptable. For example, a browser that can only accept GIF files may use Accept: image/gif.

The client specifies multiple document types that it can accept by separating the values with commas:

Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*

Some older browsers send the same line as:

Accept: image/gif
Accept: image/x-xbitmap
Accept: image/jpeg
Accept: image/pjpeg
Accept: */*

When developing a new application, it is recommended that it conform to the newer practice of separating multiple document preferences by commas, with a single Accept header.

In the server's response, the Content-type header describes the type and subtype of the media. If the client specified an Accept header, the media type should conform to the values used in the Accept header. Clients use this information to correctly handle the media type and format of the entity-body.

A client might also use a Content-type header with the POST or PUT method. Most commonly, with many CGI applications, clients use a POST or PUT request with information in the entity-body, and supply a Content-type header to describe what data can be expected in the entity-body.

Table 1.2 lists commonly used media types, along with the filename suffixes that are recognized by most servers. These servers can be easily configured to recognize additional suffixes as well.

Table 1.2. Internet Media Types

Type/Subtype	Usual Extension
application/activemessage
application/andrew-inset
application/applefile
application/atomicmail
application/cals-1840
application/commonground
application/cybercash
application/dca-rft
application/dec-dx
application/EDI-Consent
application/EDIFACT
application/EDI-X12
application/eshop
application/hyperstudio
application/iges
application/mac-binhex40
application/macwriteii
application/marc
application/mathematica
application/msword	doc
application/news-message-id
application/news-transmission
application/octet-stream	bin
application/oda	oda
application/pdf	pdf
application/pgp-encrypted
application/pgp-signature
application/pgp-keys
application/pkcs7-mime
application/pkcs7-signature
application/pkcs10
application/postscript	ai, eps, ps
application/prs.alvestrand.titrax-sheet
application/prs.cww
application/prs.nprend
application/remote-printing
application/riscos
application/rtf	rtf
application/set-payment-initiation
application/set-payment
application/set-registration-initiation
application/set-registration
application/sgml	sgm, sgml, gml, dtd
application/sgml-open-catalog	soc, cat
application/slate
application/vemmi
application/vnd.$commerce_battelle
application/vnd.3M.Post-it-Notes
application/vnd.acucobol
application/vnd.anser-web-funds-transfer-initiation
application/vnd.anser-web-certificate-issue-initiation
application/vnd.audiograph
application/vnd.businessobjects
application/vnd.claymore
application/vnd.comsocaller
application/vnd.dna
application/vnd.dxr
application/vnd.ecdis-update
application/vnd.ecowin.chart
application/vnd.ecowin.filerequest
application/vnd.ecowin.fileupdate
application/vnd.ecowin.series
application/vnd.ecowin.seriesrequest
application/vnd.ecowin.seriesupdate
application/vnd.enliven
application/vnd.epson.salt
application/vnd.fdf
application/vnd.ffsns
application/vnd.FloGraphIt
application/vnd.framemaker
application/vnd.fujitsu.oasys
application/vnd.fujitsu.oasys2
application/vnd.fujitsu.oasys3
application/vnd.fujitsu.oasysprs
application/vnd.fujitsu.oasysgp
application/vnd.fujixerox.docuworks
application/vnd.hp-hps
application/vnd.hp-HPGL
application/vnd.hp-PCL
application/vnd.hp-PCLXL
application/vnd.ibm.MiniPay
application/vnd.ibm.modcap
application/vnd.intercon.formnet
application/vnd.intertrust.digibox
application/vnd.intertrust.nncp
application/vnd.is-xpr
application/vnd.japannet-directory-service
application/vnd.japannet-jpnstore-wakeup
application/vnd.japannet-payment-wakeup
application/vnd.japannet-registration
application/vnd.japannet-registration-wakeup
application/vnd.japannet-setstore-wakeup
application/vnd.japannet-verification
application/vnd.japannet-verification-wakeup
application/vnd.koan
application/vnd.lotus-wordpro
application/vnd.lotus-approach
application/vnd.lotus-1-2-3
application/vnd.lotus-organizer
application/vnd.lotus-screencam
application/vnd.lotus-freelance
application/vnd.meridian-slingshot
application/vnd.mif
application/vnd.minisoft-hp3000-save
application/vnd.mitsubishi.misty-guard.trustweb
application/vnd.ms-artgalry
application/vnd.ms-asf
application/vnd.ms-excel
application/vnd.ms-powerpoint
application/vnd.ms-project
application/vnd.ms-tnef
application/vnd.ms-works
application/vnd.music-niff
application/vnd.musician
application/vnd.netfpx
application/vnd.noblenet-web
application/vnd.noblenet-sealer
application/vnd.noblenet-directory
application/vnd.novadigm.EDM
application/vnd.novadigm.EDX
application/vnd.novadigm.EXT
application/vnd.osa.netdeploy
application/vnd.powerbuilder6
application/vnd.powerbuilder6-s
application/vnd.rapid
application/vnd.seemail
application/vnd.shana.informed.formtemplate
application/vnd.shana.informed.formdata
application/vnd.shana.informed.package
application/vnd.shana.informed.interchange
application/vnd.street-stream
application/vnd.svd
application/vnd.swiftview-ics
application/vnd.truedoc
application/vnd.visio
application/vnd.webturbo
application/vnd.wrq-hp3000-labelled
application/vnd.wt.stf
application/vnd.xara
application/vnd.yellowriver-custom-menu
application/wita
application/wordperfect5.1
application/x-bcpio	bcpio
application/x-cpio	cpio
application/x-csh	csh
application/x-dvi	dvi
application/x-gtar	gtar
application/x-hdf	hdf
application/x-latex	latex
application/x-mif	mif
application/x-netcdf	nc, cdf
application/x-sh	sh
application/x-shar	shar
application/x-sv4cpio	sv4cpio
application/x-sv4crc	sv4crc
application/x-tar	tar
application/x-tcl	tcl
application/x-tex	tex
application/x-texinfo	texinfo, texi
application/x-troff-man	man
application/x-troff-me	me
application/x-troff-ms	ms
application/x-troff	t, tr, roff
application/x-ustar	ustar
application/x-wais-source	src
application/xml	xml, dtd
application/x400-bp
application/zip	zip
audio/32kadpcm
audio/32kadpcm
audio/basic	au, snd
audio/vnd.qcelp	wav
audio/x-aiff	aif, aiff, aifc
audio/x-wav	wav
image/cgm	cgm
image/g3fax
image/gif	gif
image/ief	ief
image/jpeg	jpeg, jpg, jpe
image/naplps
image/png	png
image/tiff	tiff, tif
image/vnd.dwg
image/vnd.dxf
image/vnd.fpx
image/vnd.net-fpx
image/vnd.svf
image/vnd.xiff
image/x-cmu-raster	ras
image/x-portable-anymap	rpnm
image/x-portable-bitmap	pbm
image/x-portable-graymap	pgm
image/x-portable-pixmap	ppm
image/x-rgb	rgb
image/x-xbitmap	xbm
image/x-xpixmap	xpm
image/x-xwindowdump	xwd
message/external-body
message/http
message/news
message/partial
message/rfc822
model/iges
model/mesh
model/vnd.dwf
model/vrml
multipart/alternative
multipart/appledouble
multipart/digest
multipart/form-data
multipart/header-set
multipart/mixed
multipart/parallel
multipart/related
multipart/report
multipart/voice-message
text/enriched
text/html	html, htm
text/plain	txt
text/richtext	rtx
text/sgml	sgm, sgml, gml, dtd
text/tab-separated-values	tsv
text/xml	xml, dtd
text/x-setext	etx
video/mpeg	mpeg, mpg, mpe
video/quicktime	qt, mov
video/vnd.vivo
video/vnd.motorola.video
video/vnd.motorola.videop
video/x-msvideo	qvi
video/x-sgi-movie	movie