We'll create fresh WordPress site with .html for all url installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Adds .html to pages, post, custom post type
1) Login your wordpress account
2) Activate .html for all plugin
2) Now go to settings->.html For All
3) Select the Post type you want to have .html extension in it and save them.
Thats all, Now just refresh your page, post or CPT and see the magic.
This Plugin closely related with rewrite rules so if anything problem seem in loading page with .html, refresh the permalink once
Go to settings->.permalink and update it.