It's a Kuwaiti Air Force C-17 transporter, the RAF has the same aircraft. Quite a large cargo aircraft that can also carry pax.
As for not divulging what it is carrying, does the site always show what's onboard? Even military aircraft have to have a manifest to fly and has to tell every country it flies over, when filing it's flight plan and requesting clearances etc, what is onboard and request dispensation if it's carrying certain types of dangerous goods.
However, this could simply be classed as a 'Diplomatic' flight and therefore the people who load and unload it won't even really know what's onboard.
Edit, or it could just be empty.