Server Side Includes

Filenames

By default, the only files that will be parsed for server-side-include tags will be those that have the extension .shtm or .shtml

You can specify additional/different extensions by placing a .htaccess file in you public_html directory, that contains the following directives:
AddType text/html .shtml .shtm
AddHandler server-parsed .shtml .shtm

This example will tell the server to parse files with the extension .shtm, .shtml, .htm, and .html:
AddType text/html .shtml .shtm .htm .html
AddHandler server-parsed .shtml .shtm .htm .html