Mentioning here for community value, couldn't find it mentioned elsewhere.
If you want to put PHP within a .html you have to use the following method:
# on the customers web server, get the AddHandler for
grep "AddHandler.*php" /var/local/enhance/apache/vhosts/$domain
# within the customers .htaccess put the addhandler as above
AddHandler "proxy:fcgi://10.169.0.117:9000" .html
# as admin within website > $domain > overrides > php-fpm add override for
security.limit_extensions > False
I'd like to see this as a feature as it is in many other panels.