It will get overwritten if you make changes to that site settings, I don't think it will get overwritten by changes to other sites. You could add it in access.conf or with a .htaccess file in that site's /web directory.
Thank-you, the .htaccess option is perfect as its portable across our different servers. For the record, this is my .htaccess entry: AddType application/x-httpd-php .html Julian