Web pages that use SSI (server side includes) by convention should be named with the .shtml file extension.
The definitive guide for SSI use with your Astutium package is http://httpd.apache.org/docs/howto/ssi.html
It is possible, using an .htaccess file to set the Server SSI pre-processor to read any kind of file (usually restricted to xhtml type files) with these entries:
Options +Includes
AddHandler server-parsed .shtml .shtm .html .htm .xhtml .xhtm
This will cause all files ending in .shtml .shtm .html .htm .xhtml and .xhtm to be parsed by the SSI Pre-Processor Engine on the server
