There has always been a debate about the advantages of having the .html extension at the end of the WordPress permalink URL.
Example: http://YourBlog.com/category/some-topic.html
What I can assure is that this concept is not that beneficial in terms of SEO in a larger persepctive. So do it for your comfort and don’t expect much of Search Engine Optimization benefits out of this trick.

As per my knowledge the most preferred form of permalink structure is this:
/%category%/%postname%
So all you need to do is change the permalink to:
/%category%/%postname%.html
Adding the extra “.html” at the end of the Permalink structure of your blog is all that it takes you to make your WordPress permalink end with the .html.
This will ensure that all of your posts for example:
http://npxp.com/npxp-stats
gets converted to
http://npxp.com/npxp-stats.html
It holds good for any permalink structure of WordPress, so just add “.html” to the end of your permalink structure of Your WordPress blog and you are good to go. This change can be performed at any point of the time after you start a blog but make sure that you have the 301 redirect enabled by using this Free WordPress plugin just in case:
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Hey, this is Kamal and I operate the NPXP network of websites. I am a blogger as well as a freelancer.
Nice post.. i changed my sufix of permalink…
Its very helpful for us.. thanks