Peel the shrimp, rinse and drain well, and use the heads and shells to make the seafood stock; refrigerate shrimp until ready to use.
Combine the seasoning mix ingredients in a small bowl and set aside.
In a 4-quart (4-liter) heavy soup pot, melt the margarine over medium heat. Add the onions, celery and bell peppers. Turn heat to high and stir in the gumbo file, Tabasco, garlic and seasoning mix. Cook 6 minutes, stirring constantly. Reduce heat to medium and stir in the tomato sauce; continue cooking 5 minutes, stirring constantly. (During this time the mixture will begin sticking to the pan bottom. As it does so, continually scrape pan bottom well with a spoon. The scrapings not only add to the gumbo's flavor, but also decrease the gumbo file's ability to thicken.)
Add the stock and bring gumbo to a boil; reduce heat and simmer 45-60 minutes, stirring occasionally.
Add the shrimp, crabmeat and oysters (if desired); cover and turn off the heat. Leave the pot covered just until the seafood is poached, about 6 to 10 minutes. Serve immediately.
For a main course, place about 1/3 cup (80ml) of rice in each bowl and top with about 1 cup (240ml) gumbo. For an appetizer, serve about half that amount.
Note: If shrimp with heads and shells are not available, use 1/2 lb (225g) shrimp without heads but with shells and substitute other seafood ingredients for the shrimp heads in making the seafood stock. You can substitute pieces of fish for any or all of the seafood in this recipe. Be sure to use margarine instead of butter, because margarine is oilier and seems to conduct more heat. The extra heat, plus the additional oil, develops the gumbo file to a more desirable taste, texture and color. Upon reaching a temperature above 140' (60'C), however, the oil separates out and rises to the surface. Some people prefer to skim the oil off before serving. If the gumbo is made in advance, do not add the seafood. When ready to serve, bring the gumbo to a rapid boil, lower the heat to a simmer, and add the seafood. Immediately cover the pot, turn off the heat, and let the pot stand covered 6 to 10 minutes.