Joomla allows you to enable or disable the adding of the .html suffix at the end of the URLs. Someone says that .html suffix help search engines to understand that it is a static page, which in turn make it more reliable. This is only one of the all Joomla SEO settings, so to know how to add the .html suffix to the end of URLs follow this steps:
Log into Joomla.
In the left menu click on System=> Global Configuration.
Choose Site tab.
Choose Yes, on the Adds Suffix to URL row, on the right part of the page.
This setting will works only if the SEF URLs setting is on.
Find more tips and tricks in Joomla tutorials section