Drupal 7 - How add posfix '.html' to all paths?
AFAIK, it is not possible to simply configure Drupal for that. If you serve your Drupal site with Apache, you can configure mod_rewrite to remove the .html suffix. But you will not be able to configure Drupal to add it to the links it produce.

In a custom module, you could implement hook_url_inbound_alter() and hook_url_outbound_alter() to remove/add to .html suffix to all paths.