July 19 (Reuters) - Portillo’s, a restaurant chain known for its Chicago-style hot dogs, said on Monday it had confidentially filed paperwork with U.S. regulators for an initial public offering (IPO). The company joins a string of restaurant brands looking to cash in on the U.S. capital market that is flush with funds. (Reporting by Sohini Podder; Editing by Sriraj Kalluvila)