MIME Types

Reference: Lincoln D. Stein, How to Set Up and Maintain a WWW Site, Addison Wesley, 1995, pp. 42-45.

MIME: Definition, Examples

Definition: MIME means Multipurpose Internet Mail Extensions

MIME was devised for Internet Mail (see RFCs 822 and 1521), to allow a mail message to carry something other than a plain text message. The use of MIME in HTTP differs in several ways from Internet Mail, "to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients." [from HTTP/1.1 spec]

The format of a MIME type (according to the HTTP/1.1 spec) is

type/subtype

"Media-type values are registered with the Internet Assigned Number Authority (IANA). The media type registration process is outlined in RFC 1590 [17]." [from HTTP/1.1 spec]

Internet media types are registered with a canonical form. Thus any client can handle a message from any server in the world.

Examples:

Type	Description
text/html	Data is HTML
text/plain	Data is plain text
text/richtext	Data is rich text format, meeting RFC1523
text/tab-separated-values	Data is tab separated text
video/mpeg	Data is MPEG video
video/quicktime	Data is Quicktime movie
audio/x-wav	Data is Microsoft "wav" format audio file
image/gif	Data is GIF image
image/tiff	Data is TIFF image
application/msword	Data is Microsoft Word file
application/rtf	Data is Microsoft rich text format
aplication/zip	Data is compressed with PKZip
message/rfc822	E-mail message, Internet format
multipart/mixed	contains multiple MIME types

Rules To Set MIME Type

How MIME type is set:

• For documents sent by HTTP server: server sets MIME type in HTTP header:
  • Server could list URL in a table that specifies MIME type (so a .gif file could be given MIME type of text/plain!)
  • Server looks up other URL's file name extension (e.g., .ps) in table on server.
  • If URL is script, then script sets MIME type.

• For documents sent by other servers (FTP, GOPHER): client sets MIME type based on table in client

Helper Applications for Web Clients

Web browsers can present only certain MIME types; for rest, they either

• Prompt user to save document
• Prompt user for application to run
• Use a helper application (user can add new ones)

In Netscape 3.0, select the Options menu, then General preferences... to display the following screen:

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Last modified on 10 September 1996.

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