If you followed my instruction about permalinks you need 2 more plugins to finish the job.
.HTML on Pages
The .HTML on pages plugin automatically append .html to the wordpress “page”(s).
The permalink trailing slashes fixer completes the job adding a trailing slash to category URLs and other links that does not ends with .html.
NOTE: this plugin require a little patch, see here
Top level categories
The top level categories plugin allow you to avoid them to appear under the root of your URL instead of under the “/category” path.