How to make a redirect from a link without .html to a link with .html without affecting the categories?

Question

htaccess

Tell me how to make redirects to a link ending in .html, but so that the categories remain /category/ ?
I found this rule
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^.]+)$ /$1.html [R=301,L]
But it adds .html to categories as well