A short

Its an short circuit at very high frequencies. Look at the impedance for a capacitor and you'll notice that the frequency is inversely related to it. (also note that capacitor impedance is purely reactive). Therefore, as frequency increases the impedance approaches 0, which acts like a short. (an inductor is the opposite)

Initially as the frequency goes up, the impedance goes down, however eventually it will bottom out at a resonance point. Past this, it looks like an inductor. At very high frequencies, the parasitic inductance completely takes over and the capacitor becomes inductive.

An open