Place the first 11 ingredients in a stockpot; bring to a boil. Reduce heat; cover and simmer for 1-1/2 hours., Remove chicken and allow to cool. Strain broth, discarding vegetables; skim fat. Remove meat from bones; cut chicken into bite-size pieces and set aside. Discard bones., In the same pan, cook and stir oil and flour over medium heat until caramel-colored, about 14 minutes (do not burn). Add finely chopped onion; cook and stir 2 minutes longer. Gradually stir in broth. Bring to a boil., Carefully stir in sausage and reserved chicken. Reduce heat; simmer, uncovered, for 10 minutes. Stir in okra and oysters. Simmer, uncovered, 10-15 minutes longer or just until okra is tender. Stir in file powder. Serve with rice.