File (Fee-lay) is considered a substitute for okra in gumbo. File is ground sassafras leaves. It's usually sold in the spice section under the name "gumbo file".
"The Creoles liked the delicate flavor and began using it in their soups and stews. They found that besides adding flavor, file also thickened with the same slippery smoothness of okra, and could be substituted when okra wasn't in season" -- The Little Gumbo Book