Slow Cooker Smoked Chicken and Shrimp Gumbo

Name: Slow-Cooker Smoked Chicken and Shrimp Gumbo
Ingredients: 6 tablespoons unsalted butter; 1 tablespoon liquid smoke; 1/2 cup all-purpose flour; 1 medium white onion, finely chopped; 1 green bell pepper, finely chopped; 2 stalks celery, finely chopped; 2 cloves garlic, minced; 2 1/4 pounds skinless, bone-in chicken thighs, trimmed of excess fat; 6 cups low-sodium chicken broth; 3 tablespoons Cajun seasoning; 1 tablespoon file powder (gumbo seasoning, such as Zatarain's); 1 1/2 pounds medium shell-on shrimp; Cooked white rice and sliced scallions, for serving; Hot sauce, for serving