Custom 404 Question

Is there a way to get a custom 404 message to come up when a .html, .htm, or .asp page is requested on discountASP.net? I've got it working (via web.config) for .aspx pages, but I cannot find a reference on how to get it to work with .html pages.

We have recently converted my company's website from straight html and asp to asp.net and do not want to alienate old users with links to html pages.

Thanks in advance for everyone's help.
-Paul