01Heat the oil in a heavy 5-quart pot over medium heat. Add the chicken and brown on all sides. Remove chicken and set aside.
02Make a roux by blending the flour into the oil in the pot and stirring constantly with a long-handled spoon until the mixture is a deep coffee brown. Add the sausage, ham, onion, green pepper and celery. Saute, stirring, for about 10 minutes. Add the browned chicken, scallions, garlic, parsley, thyme, bay leaves and enough water to cover, stirring to incorporate the roux into the liquid.
03Stir in salt and pepper and simmer the soup slowly, partly covered, until the chicken is tender, about 1 hour. If you are using the very spicy rind of tasso ham, you will probably not need cayenne or Tabasco. Otherwise, add either toward the end of cooking.
04Take out the chicken and set aside just until cool enough to handle. Pull off the skin and bones and return the chicken to the soup in large pieces. Adjust seasoning if needed. The soup can be prepared in advance up to this point and stored, covered, in the refrigerator for 2 days. Skim fat from soup.
05Bring the soup to a simmer. Five minutes before serving, remove from heat and stir in the file powder. Cover and let stand for about 5 minutes. Do not boil the soup after the file has been added. Mound rice in soup plates and ladle the gumbo over.