The header will specify a file's type. The list is hardly exhaustive of course, and in fact web servers and web browsers can easily extend the MIME specification to add new MIME types. All you need to do is make sure that the server is configured to output the new type and that browsers are configured to expect the new type. Getting browsers to understand new types is usually a function of the options or preferences settings in that browser. Consider the Netscape preferences dialog.
MIME type is specified in the Content-type field.
Content-encoding Specifies that the message body is encrypted, compressed, or encoded. Content-type Indicates the media type of the information in the message body. Content-length Specifies the number of bytes in the message body for requests with a message body. Date Indicates the date and time of a request if the request has a message body. Mime-version Specifies the MIME version used to generate the message body.