MIME Types MIME types are pre-defined file types with defined file extensions making it possible to transport file types via the World Wide Web or by e-mail. A MIME Type tells the web server that a .html document is a web page, and that a .gif doc is a gif image etc. If an extension is not stored within the Mime Type, the web server will not recognize it and that particular file type will not be viewable by the user. This feature is not available for FrontPage users.