It is xhtml, looks like they have just used a sound file extension .sf (not sure why though)? I am not a coder so maybe someone who knows better will comment.
you can call your filenames anything and tell the browser to parse it as whatever, in the HTACCESS.
My unbrella consultancy website shows htm/html extensions, but it is actually php. I didn't want to redirect when I redesigned using php etensions includes etc, so I just rewrote using htaccess.
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