Like most of the revelations and allegations that have poured out of Andrew M. Cuomo's office in the New York attorney general's investigation into the student loan industry, his latest is provocative and highly charged. Two weeks ago, in a powerfully worded letter to Sen. Christopher Dodd (D-Conn.) and Rep. George Miller (D-Calif.), Cuomo accused some providers of private student loans with engaging in the kind of racial discrimination that home mortgage lenders once practiced.