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Media types

An Internet media type is, generally speaking, a property of a data set, describing both the general type of data (such as "text" or "image" or "application"; the last one refers to program-specific internal data formats) and, as a subtype, a specific format for the data. The concept was originally defined as "MIME content types".

Media types relate to HTML as follows:

  • When a Web server sends an HTML document, it should specify the correct media type (text/html) in the HTTP headers it sends along with the document. Normally servers are configured to do this by default when the file name ends with .html or .htm (depending on the system; please consult local documentation).
  • In a FORM element, the value of the ENCTYPE attribute specifies the media type to be used then encoding and sending the content of the form.
  • When referring to various resources, such as embedding images using IMG elements or linking to binary files using an A element, there is no way to tell the media type in HTML. Things must be handled in the server. Typically, a Web server uses some mapping table to map file name extensions to media types (eg mapping extension .zip to media type application/zip), and it may provide users some tools for overriding such mappings or otherwise specifying the media type to be associated with a file or set of files. The description of the A element contains some additional notes related to audio and video and binary files in general.

The HTML 3.2 Reference Specification refers to RFC 1521 but that specification was superseded by RFC 2046 (in November 1996). The procedure for registering types in given in RFC 2048; according to it, the registry is kept at ftp://ftp.isi.edu/in-notes/iana/assignments/media-types/

For less authoritative but more readably presented information, see document MIME Types by Chris Herborth.

In addition to standardized media types, there are media types which are in fact supported by popular servers and browsers. Appendix B of Special Edition Using CGI lists many of them.

You can check what is the media type information sent by a server as follows: Assuming we are interested in the media type of the document at URL http://host/path, establish a Telnet connection to host using the port number in the URL if present, port 80 otherwise. Then give the command
HEAD /path HTTP/1.0
and then an empty line. Example (where the Telnet connection is established by starting the telnet program from Unix command level):

beta ~ 51 % telnet www.hut.fi 80
Connected to info-e.hut.fi.
Escape character is '^]'.
HEAD /home/jkorpela/perhe.jpg HTTP/1.0
HTTP/1.1 200 OK
Date: Tue, 23 Sep 1997 12:37:05 GMT
Server: Apache/1.2.4
Last-Modified: Tue, 08 Aug 1995 08:29:53 GMT
ETag: "16391-9232-30272081"
Content-Length: 37426
Accept-Ranges: bytes
Connection: close
Content-Type: image/jpeg
Connection closed by foreign host.
beta ~ 52 % exit
Here the Content-Type: field tells that the media type is image/jpeg.

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