Prepare rice as per package directions.
Combine oil and flour in a large heavy saucepan. Cook, stirring frequently, over medium heat until mixture become milk chocolate colored. Add onion, celery and bell pepper. Sauté 2 minutes. Add chicken and Cajun seasoning and sauté 5 minutes. Add chicken broth and tomatoes. Simmer 20 minutes. Stir in file powder, if desired. Ladle into bowls and top with rice and hot sauce, if desired.