Change cached file name

FlyingPress generate a default cache file name like "index.html" or "index-logged-in.html", etc.

The " flying_press_cache_file_name" filter is used to modify the name of the cache file generated by FlyingPress.

add_filter('flying_press_cache_file_name', function($filename){
   $filename = 'joy-'.$filename;
   return $filename;
});