If you are looking for the optimum SEO format for your filenames, you’ll want to set this up pretty early so you don’t got and change it later. If your wordpress site is already ranking and you have pages getting indexed, then this permalinks solution will cause errors on those pages within the search engine
Now the reason for adding a filename extension is due merely to a posting on Google where Matt Cutts says it’s better to have files with extensions for a more natural indexing. Meaning, that you might increase your chances slightly.
Again if your site has been around for some time, I’d pass on this little SEO tweak. Fro new sites, you’ll want to follow accordingly.
01- Login in your WordPress admin
02- Click Permalinks below the Settings tab
03– In permalinks page you will see your WordPress default permalinks structure is /%category%/%postname%/
04– Now just add .html to that structure.
So it will look like /%category%/%postname%.html
05 – Good to go, now SAVE
SEO TIP: even better, leave out the /%category%/ portion in permalinks. It should ultimatelly look like this:
this keeps it limited to domain name and post title only
Resulting in this:
How to add a file extension to your WordPress posts?