WARSAW, Dec 16 (Reuters) - Polish security services have detained a 19-year-old student suspected of seeking to establish contacts with Islamic State and charged with preparing an attack at a Christmas market that could have caused mass casualties, authorities said on Tuesday. Mateusz W., a student at the Catholic University of Lublin, wanted to commit an attack using explosives and planned to join a terrorist organization, said Jacek Dobrzynski, a spokesperson for Poland's special services. "The man was very fascinated by Islam, sought to establish contacts with the Islamic State, and was preparing an attack in Poland, in one of...