Place flour in a 15- x 10- x 1-inch jellyroll pan. Bake at 400° for 10 to 15 minutes or until flour is caramel colored, stirring every 5 minutes. Set aside.
Peel and devein shrimp. Set aside.
Combine browned flour, tomato, and next 4 ingredients in a large Dutch oven, stirring well. Bring to a boil; cover, reduce heat, and simmer 15 minutes. Add vegetable gumbo mixture; cover and cook 20 minutes. Add shrimp and oysters; cook 5 minutes or until shrimp turn pink and oyster edges curl. Remove from heat, and sprinkle with file powder, if desired.