File powder is made from the dried, ground leaves of the sassafras tree. It was used by the Choctaw people of Louisiana long before European settlement, and was one of the first indigenous ingredients adopted into Creole cooking. File contributes two things to gumbo: a subtle, earthy flavor and thickening power.
When to Add It
File powder must be added off the heat -- after the gumbo has finished cooking and has been removed from the burner. Add it to boiling liquid and it becomes stringy, slimy, and bitter. This is one of the most common mistakes made by cooks encountering file for the first time.
The correct method: remove the pot from heat, let it rest for 2-3 minutes, then stir in the file powder. Cover and let stand for 5 minutes before serving. The file will thicken the gumbo slightly as it sits.
File and Okra
Traditionalists argue that you should use either file OR okra as a thickener, not both. Okra thickens gumbo during cooking through its natural mucilage. File thickens after cooking. Using both makes for an overly thick, somewhat gluey gumbo. Choose one approach and stick to it -- though many modern Cajun cooks use a small amount of both with good results.