In a large pot, heat the oil over medium heat.
Add the flour and stir constantly for 20–25 minutes until the roux becomes a dark brown color.
Add the chopped bell pepper, celery, onion, and garlic to the pot.
Cook until the vegetables are soft, around 5 minutes.
Stir in the chicken broth, water, and canned tomatoes.
Bring the mixture to a gentle simmer.
Add the shrimp and crabmeat.
Let the gumbo simmer for about 30 minutes, stirring occasionally.
If desired, stir in the file powder and let the gumbo stand for 5 minutes before serving.