MIME Types

By John Biswell on 3rd March, 2011.

About MIME Types

Multipurpose Internet Mail Extensions (MIME) is a specification for creating file formats to be used in the exchange of e-mail, in Web documents, and in other applications on intranets and on the Internet. Each MIME format includes a MIME content type and subtype which denote the kind of data stored in the file.

MIME types and subtypes are typically listed as type/subtype. For example, an MPEG video file would be listed as “video/mpeg.” Type/subtype listings include acceptable file name extensions. For example, a table listing MIME information for an MPEG file might have an MPEG entry that appears as follows:

Type/Subtype: video/mpeg

Extensions: .mpe, .mpeg, .mpg

MIME and the Web

MIME-formatted content is widely used in Web documents. For example, you could link an MPEG file to a Web page. When a user viewing the page clicks on the MPEG-file link, Internet Information Services (IIS) and the Microsoft® Internet Explorer browser perform the following sequence of events when downloading the MPEG file to the browser for display:

Along with the file, IIS sends the MIME type and subtype (video/mpeg, video/mpe, or video/mpg) to the browser.

If support for the MPEG MIME format is built-in, the browser displays the file.

If the browser does not contain built-in support for the MIME format, the browser looks up the file name extension in its table of helper applications. Then it either selects the appropriate helper application to display the file or returns an error message.

ISS Table

IIS includes a table of the most common Web-related types/subtypes and associated file name extensions. If IIS does not have MIME information for a MIME-formatted file that you download, then IIS will attach a default MIME identity to that file. As a result, the client receiving the file may misinterpret its contents. You can manually add MIME types, subtypes, and file name extensions to IIS for files whose MIME identity is not in the table. See the section on adding MIME types which applies to your version of IIS.

Note that when planning to use a MIME format, you must configure the browser for a helper application to process any MIME type not supported directly by the browser.

John Biswell

Operations Director

John is Operations Director at Secura and a Windows Server expert. Before all things IIS related, he was also a champion powerlifter.

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