User promted to download file for 3rd party page

I have a page that uses a third party booking site in a frame. When a user visits that page now they are prompted to download a file. Can you tell me why this is happening and what I can do to fix it? The page is http://www.vmcleodlaw.com/online-booking-calendar/

http://wordpress.org/extend/plugins/w3-total-cache/