Normally it is required by system defaults that any webpage using SSI have an extension of shtml.

However, if you have a friendly ISP.
And, if you can do a little htaccess work.
And, if all or almost all of your pages have SSI.
Then, you can reset the expected extention which the server daemon will process as SSI.

Look under htaccess code snippets for more on this.

But, as I recall you can use something like:
Options Includesnoexec
AddType text/x-server-parsed-html .html