Adding post ID and .html to permalinks

A site I have just become involved with recently lost all facebook likes and tweet stats after updating wordpress. After digging through some old links they’d posted to twitter I found that the post permalinks used to have the postname and .html at the end.

I’m guessing they had some kind of custom code in the htaccess file or elsewhere that got overwritten. Any ideas of how I could do this?

Basically I need to add “_(POST ID).html” to the end of every post.