MIME Types

Multipurpose Internet Mail Extensions

MIME types are used by web servers and web browsers. Each will contain a file that has a table of MIME types with the associated file extension for that type.

MIME type file on Apache

text/html  	html htm HTML 
text/plain 	txt
image/gif	gif
image/jpeg	jpg jpeg
...

MIME types are universal

It is the MIME type that is universal, not the extension. All systems have agreed to use MIME types to identify the content of a file transmitted over the web. File extensions are too limiting for this purpose. Many different word processor programs might use the extension .doc to identify a file. For instance, .doc might refer to a MS WORD document or to a MS WORDPAD document. It is impossible to tell from the extension which program actually created the program. In addition, other programs could use the .doc extension to identify a program: for instance, Word Perfect could also use the .doc extension. Using the extension would be too confusing to identify the content of the file.

By using a mime type, more information can be conveiged than in an extension. There are two parts to a MIME type: the general category, and the specific type. There can be several specific types in a category: image/gif, image/jpeg, image/png.

When a file is sent over the internet, it's content is identified by a content type.

Examples of using MIME types

Click on these links to see the message you get for these files:

• junk.hdf You will most likely see a dialog box like the one on the right. There is a radio list of options for handling this type of file. There is also a checkbox to indicate that this action should always be chosen.

  Notice that the option for saving it to disk is chosen, since the browser doesn't know what to do with this Mime type.

  This is the process that occurred.

  1. The browser requested the file junk.hdf from the server.
  2. The server looked up the file extension in it's MIME types file, finding a reference like this
     application/x-hdf hdf
  3. The server sends the file to the browser, identifying it's content as application/x-hdf
  4. The browser receives the file, looks in it's MIME types file, but does not have an entry for application/x-hdf, so the browser queries the user to determine what to do with the file.
  
  
• junk.doc You will probably see a dialog box like the one on the right, unless you have the browser automatically open the application to view this file.

  Notice that the option for opening with the default application is checked.

  This is the process that occurred.

  1. The browser requested the file junk.doc from the server.
  2. The server looked up the file extension in it's MIME types file, finding a reference like this
     application/msword doc
  3. The server sends the file to the browser, identifying it's content as application/msword
  4. The browser receives the file, looks in it's MIME types file and finds that application/msword has been associated with the Word program on my system.

• junk.foobar This file has an extension that the server does not recognize, so the default MIME type of text/plain is sent to the browser. The browser is able to display this, so you will see it in the browser window.
  This is the process that occurred
  1. The browser requested the file junk.foobar from the server.
  2. The server looked up the file extension in it's MIME types file, but didn't find an entry for the foobar extension. So, the server identifies the file as text/plain, since that is the default type that was set in the server's configuration file.
  3. The server sends the file to the browser, identifying it's content as text/plain.
  4. The browser receives the file, looks in it's MIME types file, and sees that text/plain can be handled by the browser, so the contents of the file are displayed.

For more MIME information, check this link: http://www.oac.uci.edu/indiv/ehood/MIME/MIME.html

For a list of mime types, check this link: ftp://ftp.isi.edu/in-notes/iana/assignments/media-types/media-types