Square Inc., the mobile-payments company founded and led by Jack Dorsey, the interim chief executive officer of Twitter Inc., filed confidentially for an initial public offering, people with knowledge of the matter said. Square, which processed $30 billion in payments from its millions of merchant customers in 2014, has been expanding into other areas such as business lending and payroll processing to provide more services. Aaron Zamost, a spokesman for San Francisco-based Square, declined to comment.