Jump to content

Media type

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by 130.76.64.109 (talk) at 23:34, 25 March 2013 (Undid revision 546765440 by 71.233.192.102 (talk)). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

An Internet media type[1] 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,[2] experimental or non-standard[3] 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;[4] subtypes that begin with prs. are in the personal or vanity tree.[5] 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.

Limitations

Internet media types are often used as part of a communication protocol between two applications (the source and destination). In this context, internet media type specifiers experience several problems.

The first problem is the ability of the source application (i.e. web server, email client) to correctly determine an internet media type for a piece of content. Many applications attempt to heuristically classify a file using its filename extension or with magic numbers. Neither approach is perfect, and may incorrectly classify a content's media type:

  • Incorrect filename extension: a filename extension classifier will report an incorrect media type. For instance, some applications incorrectly give Rich text format files the .doc file extensions, instead of the correct .rtf extension.
  • No filename extension: a filename extension classifier will report no media type, or will (incorrectly) report a catch-all type such as application/octet-stream. Files without extension are common on unix systems.
  • Filename extension collisions: when multiple formats use the same filename extension, a filename extension classifier will choose one media type arbitrarily. For instance, both Microsoft Word templates and graphviz graph files use the extension .dot.
  • Ambiguous container formats: a magic number classifier may give a correct, though non-specific, media type, thus preventing a meaningful interpretation of the content. For instance, Office Open XML (.docx) format and Java executable (.jar) are both implemented internally as a zipped archive. A magic number system may classify such files as application/zip instead of the more specific type. Similar problems occur between XML and application formats implemented on top of XML.
  • Ambiguous magic numbers: an attacker can create a file which is identified simultaneously as two separate internet media types. For instance, the internal structure of a Gifar makes it both a valid GIF image and Java executable.

The second problem is the destination application's ability to trust the internet media type reported by the sender. As above, the internet media type is incorrect in some circumstances, and must be treated with skepticism. As early as 2002, the W3C unambiguously warned that it is a "serious error" if internet media type is incorrect, and that software should not attempt to guess a correct media type.[1]: Section 2  Nonetheless, software engineering principles encourage software that forgives a certain degree of malformed input, and user experience suffers when software fails to correctly interpret the content. Consequently, the many destination applications are designed to attempt recovery from such errors and identify a correct media type.[6][7]

The destination application has no more knowledge of the content than the source application, and attempts to infer the media type at the destination are equally difficult. This can lead to incompatibilities between source and destination applications, and in the worst-case, security vulnerabilities such as the Gifar attack or Cross-site scripting attacks.[8][9] Advanced content sniffing approaches have been proposed to balance interoperability and security in such situations.[7]

List of common media types

IANA manages the official registry of media types. It includes the following types:

Type application

For Multipurpose files:

  • application/atom+xml: Atom feeds
  • application/ecmascript: ECMAScript/JavaScript; Defined in RFC 4329 (equivalent to application/javascript but with stricter processing rules)
  • application/EDI-X12: EDI X12 data; Defined in RFC 1767
  • application/EDIFACT: EDI EDIFACT data; Defined in RFC 1767
  • application/json: JavaScript Object Notation JSON; Defined in RFC 4627
  • application/javascript: ECMAScript/JavaScript; Defined in RFC 4329 (equivalent to application/ecmascript but with looser processing rules) It is not accepted in IE 8 or earlier - text/javascript is accepted but it is defined as obsolete in RFC 4329. The "type" attribute of the <script> tag in HTML5 is optional. In practice, omitting the media type of JavaScript programs is the most interoperable solution, since all browsers have always assumed the correct default even before HTML5.
  • application/octet-stream: Arbitrary binary data.[10] Generally speaking this type identifies files that are not associated with a specific application. Contrary to past assumptions by software packages such as Apache this is not a type that should be applied to unknown files. In such a case, a server or application should not indicate a content type, as it may be incorrect, but rather, should omit the type in order to allow the recipient to guess the type.[11]
  • application/ogg: Ogg, a multimedia bitstream container format; Defined in RFC 5334
  • application/pdf: Portable Document Format, PDF has been in use for document exchange on the Internet since 1993; Defined in RFC 3778
  • application/postscript: PostScript; Defined in RFC 2046
  • application/rdf+xml: Resource Description Framework; Defined by RFC 3870
  • application/rss+xml: RSS feeds
  • application/soap+xml: SOAP; Defined by RFC 3902
  • application/font-woff: Web Open Font Format; (candidate recommendation; use application/x-font-woff until standard is official)
  • application/xhtml+xml: XHTML; Defined by RFC 3236
  • application/xml: XML files; Defined by RFC 3023
  • application/xml-dtd: DTD files; Defined by RFC 3023
  • application/xop+xml:XOP
  • application/zip: ZIP archive files; Registered[12]
  • application/gzip: Gzip, Defined in RFC 6713

Type audio

For Audio.

  • audio/basic: mulaw audio at 8 kHz, 1 channel; Defined in RFC 2046
  • audio/L24: 24bit Linear PCM audio at 8–48 kHz, 1-N channels; Defined in RFC 3190
  • audio/mp4: MP4 audio
  • audio/mpeg: MP3 or other MPEG audio; Defined in RFC 3003
  • audio/ogg: Ogg Vorbis, Speex, Flac and other audio; Defined in RFC 5334
  • audio/vorbis: Vorbis encoded audio; Defined in RFC 5215
  • audio/vnd.rn-realaudio: RealAudio; Documented in RealPlayer Help[13]
  • audio/vnd.wave: WAV audio; Defined in RFC 2361
  • audio/webm: WebM open media format

Type image

Type message

  • message/http: Defined in RFC 2616
  • message/imdn+xml: IMDN Instant Message Disposition Notification; Defined in RFC 5438
  • message/partial: Email; Defined in RFC 2045 and RFC 2046
  • message/rfc822: Email; EML files, MIME files, MHT files, MHTML files; Defined in RFC 2045 and RFC 2046

Type model

For 3D models.

Type multipart

For archives and other objects made of more than one part.

  • multipart/mixed: MIME Email; Defined in RFC 2045 and RFC 2046
  • multipart/alternative: MIME Email; Defined in RFC 2045 and RFC 2046
  • multipart/related: MIME Email; Defined in RFC 2387 and used by MHTML (HTML mail)
  • multipart/form-data: MIME Webform; Defined in RFC 2388
  • multipart/signed: Defined in RFC 1847
  • multipart/encrypted: Defined in RFC 1847

Type text

For human-readable text and source code.

  • text/cmd: commands; subtype resident in Gecko browsers like Firefox 3.5
  • text/css: Cascading Style Sheets; Defined in RFC 2318
  • text/csv: Comma-separated values; Defined in RFC 4180
  • text/html: HTML; Defined in RFC 2854
  • text/javascript (Obsolete): JavaScript; Defined in and obsoleted by RFC 4329 in order to discourage its usage in favor of application/javascript. However, text/javascript is allowed in HTML 4 and 5 and, unlike application/javascript, has cross-browser support. The "type" attribute of the <script> tag in HTML5 is optional and there is no need to use it at all since all browsers have always assumed the correct default (even in HTML 4 where it was required by the specification).
  • text/plain: Textual data; Defined in RFC 2046 and RFC 3676
  • text/vcard: vCard (contact information); Defined in RFC 6350
  • text/xml: Extensible Markup Language; Defined in RFC 3023

Type video

For video.

List of common media subtype prefixes

Prefix vnd

For vendor-specific files.

Prefix x

For non-standard files.

  • application/x-deb: deb (file format), a software package format used by the Debian project
  • application/x-dvi: device-independent document in DVI format
  • application/x-font-ttf: TrueType Font No registered MIME type, but this is the most commonly used
  • application/x-javascript:
  • application/x-latex: LaTeX files
  • application/x-mpegURL: .m3u8 variant playlist
  • application/x-rar-compressed: RAR archive files
  • application/x-shockwave-flash: Adobe Flash files for example with the extension .swf
  • application/x-stuffit: StuffIt archive files
  • application/x-tar: Tarball files
  • application/x-www-form-urlencoded Form Encoded Data; Documented in HTML 4.01 Specification, Section 17.13.4.1
  • application/x-xpinstall: Add-ons to Mozilla applications (Firefox, Thunderbird, SeaMonkey, and the discontinued Sunbird)
  • audio/x-aac: .aac audio files
  • audio/x-caf: Apple's CAF audio files
  • image/x-xcf: GIMP image file
  • text/x-gwt-rpc: GoogleWebToolkit data
  • text/x-jquery-tmpl: jQuery template data

Prefix x-pkcs

For PKCS standard files.

  • application/x-pkcs12: p12 files
  • application/x-pkcs12: pfx files
  • application/x-pkcs7-certificates: p7b files
  • application/x-pkcs7-certificates: spc files
  • application/x-pkcs7-certreqresp: p7r files
  • application/x-pkcs7-mime: p7c files
  • application/x-pkcs7-mime: p7m files
  • application/x-pkcs7-signature: p7s files

See also

References

  1. ^ a b "Internet Media Type registration, consistency of use". W3C. 2002-09-04. Retrieved 2012-02-29.
  2. ^ "RFC 6648 - Deprecating the "X-" Prefix and Similar Constructs in Application Protocols". IETF. 2012. Retrieved 2012-10-07. {{cite web}}: Unknown parameter |month= ignored (help)
  3. ^ Freed, N.; Borenstein, N. (1996). "RFC 2045 - Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies". IETF. Retrieved 2006-11-29. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  4. ^ Freed, N.; Klensin, J.; Postel, J. (1996). "RFC 2048 - Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures, Section 2.1.2 - Vendor Tree". IETF. Retrieved 2011-12-05. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  5. ^ Freed, N.; Klensin, J. (2005). "RFC 4288 - Media Type Specifications and Registration Procedures". IETF. Retrieved 2008-06-14. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  6. ^ "MIME Type Detection in Windows Internet Explorer". Microsoft. Retrieved 2012-07-14.
  7. ^ a b http://mimesniff.spec.whatwg.org/ MIME Sniffing Standard, Living Standard — Last Updated 29 November 2012. Editors Gordon P. Hemsley, Adam Barth, Ian Hickson.
  8. ^ "CVE-2008-5343 (under review)". MITRE Corporation. 4 December 2008. Retrieved 1 January 2013.
  9. ^ Henry Sudhof (11 February 2009). "Risky sniffing: MIME sniffing in Internet Explorer enables cross-site scripting attacks". The H. Retrieved 2012-07-14.
  10. ^ RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media types. Tools.ietf.org. Retrieved on 2010-09-29.
  11. ^ W3C (1999). "RFC 2616: 7. Entity". Hypertext Transfer Protocol -- HTTP/1.1. The Internet Society. Retrieved 28 May 2012. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: numeric names: authors list (link)
  12. ^ MIME SUBTYPE NAME: zip
  13. ^ "Supported Media Formats". RealPlayer Help. RealNetworks. 2010. Retrieved 28 May 2012.
  14. ^ MIME SUBTYPE NAME: png
  15. ^ MIME subtype name : Vendor Tree - vnd.microsoft.icon
  16. ^ Quicktime
  17. ^ vnd.oasis.opendocument.text
  18. ^ vnd.oasis.opendocument.spreadsheet
  19. ^ vnd.oasis.opendocument.presentation
  20. ^ vnd.oasis.opendocument.graphics
  21. ^ "Application Media Types". IANA. Retrieved 2012-02-19.
Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Media_type&oldid=546989541"