lucy24 - 8:26 am on May 8, 2013 (gmt 0)
AddOutputFilter INCLUDES .html .php
(substitute the extensions you actually use)
That's assuming you'll be adding include files to everything. Option B is the x-bit hack which involves setting permissions in individual documents. It isn't very complicated in other words, I've done it myself but requires a little more work than the global version.
<!--#include file="includes/leftnav.html" -->
<!--#include virtual="includes/leftnav.html" -->