What is Gumbo file?

Gumbo file is ground Sassafras leaves. It adds thickening and flavor but should only be added to the hot gumbo shortly before serving and never boiled. If cooked very long it will turn gummy. If the pot of gumbo will get re-heated later it might be best to add just to the portion being served. About a teaspoon per serving according to my taste. Since okra adds thickening, it isn't usual to also add file, nor is it usual to cook tomatoes into the roux, at least in my family. Tomatoes are often added to okra to reduce the gumminess. I have not tried this recipe.