The popular Hank Williams song from the 'fifties goes, "Jambalaya, crawfish pie,..., file gumbo..." The famous file powder (pronounced [fee-lay]) from the song refers to a spice made from the dried, ground leaves of the sassafras tree. File powder is a key ingredient for Cajun and Creole cuisine, especially gumbo, where it is added as a seasoning and thickening agent.
Generations before the Cajuns arrived in Louisiana, Choctaw indians were using the ground leaf of the native sassafras tree in their soups and stews. File powder smells like eucalyptus or Juicy Fruit gum, and has a distinctive "root beer" flavor. The earthy taste of file powder is similar to thyme combined with savory.
File powder will lend a unique flavor to stews, sauces and other hearty dishes. Use file powder when you don't have [or don't like] okra, as this magic powder will thicken soup or sauce in the same way. Sprinkle file powder sparingly over gumbo as a seasoning and a thickening agent, stirring it in at the end of your cooking. Don't let the file powder come to a boil -- it will cause the broth to become stringy.
Some Cajun cooking enthusiasts pass little bottles of file powder and Tabasco at the dinner table, and let guests sprinkle both to their hearts' content.