Okra or cornstarch can be substituted for file powder in gumbo. Both substitute as a thickening agent, but do not impart the same distinctive flavor as the file powder.
Most gumbo recipes use two types of thickeners: a roux and either file powder or okra. File powder is ground sassafras leaves and adds a distinctive root beer flavor to gumbo. If okra is used, it should be cooked for approximately 45 minutes to lose its slimy texture, while file powder should be added at the end of cooking. It is recommended that okra and file should not be used in the same batch of gumbo, as the gumbo may become too thick.