.htm and underscores on PAGES
Appends .htm to the URL of PAGES when using permalinks and swaps dashes with underscores
Appends .htm to the URL of PAGES when using pretty permalinks and swaps dashes with underscores
Check out more information here http://devcreations.com/wordpress-plugins/htm-n-underscores-on-pages.html
DISCLAIMER: This plugin has been heavily tested, however it does require above average technical knowledge. Please use it at your own risk and do not forget to back up your files and databases beforehand with other backup system. Please do not attempt to use the plugin if you're new to WordPress or have a limited technical background. Please seek out professional help if your in question of anything.