It's almost exactly the same principle as redirecting with-or-without www. to your preferred format. If you really have a significant number of people aiming for the wrong extension, add a general \.html to \.htm redirect. And, as with all other generic redirects, make sure your own links-- and any others that you have control over-- are correct.
:) I knew someone would suggest that-- but here we seem to be talking about pages that already exist, so you wouldn't exactly be saving yourself any trouble. More work in fact, since you first have to do a redirect to put existing links into the new format, and then a rewrite to fetch contact from where it really lives.
But then, my automatic reaction to an extensionless url is to tell it to go put some clothes on ;)