How does this circuit attenuate high frequencies?
The AC feedback via C2 is negative feedback - it will decrease the gain.
 Increasing the negative feedback with frequency will reduce the gain as frequency increases.
 C2 impedance decreases as frequency increases so feedback via C2 increases so gain decreases if all feedback is applied to inverting input. 

Final result will depend on where the feedback goes / how it is applied. Changing R3 split changes the overall "transfer function". 

It's not obvious that this is a formal design. Can you give a reference to where you got it from?