Files always cached with .php at the end of the slug

Not sure if this is the correct way that the plugin works but all my urls in the “Fresh Super Cached Files” have a “.php” added to the end.

http://www.domain.com/google-unveil-their-new-futuristic-glasses/.php

Obviously URLS give a 404 error.

When i visit the supercached folder on my server the file is called:

index.html.php

is this correct?

http://wordpress.org/extend/plugins/wp-super-cache/