As I understand it, Drupal is supposed to check the web root for any URLs it doesn't recognize, to see if it's a non-Drupal file on disk. Also, Apache is supposed to assume that anything without a file extension is a request for whatever.html, correct?
Here's what I'm trying to achieve: a user requests mysite.com/FOO, Drupal doesn't find any path/alias for FOO, and thus turns it over to Apache. Apache doesn't see a file extension, and thus looks for a file called FOO.html (which exists) and serves up that file.
If I request mysite.com/FOO.html, the file is returned just fine. I'm just trying to make it so that people can access FOO.html by simply requesting /FOO, since you can't trust the user to type in the full URL. (And because it looks nicer in a print ad.)
I'm guessing that there's some .htaccess magic that needs to happen, but I haven't been able to come up with the right trick. I think that Drupal passes to Apache, which doesn't find /FOO, then the .htaccess file punts the request to Drupal's index.php, which returns the Drupal 404 page.