Slow Cooker Shrimp and Chicken Gumbo

Name: Slow-Cooker Shrimp and Chicken Gumbo
Ingredients: 2 lb. (1 kg) skinless, bone-in chicken thighs; Kosher salt and freshly ground pepper; 3 Tbs. unsalted butter; 3 Tbs. canola oil; 1/2 cup (2 1/2 oz./75 g) all-purpose flour; 1 yellow onion, finely chopped; 1 small green bell pepper, seeded and diced; 1 small red bell pepper, seeded and diced; 1 large celery stalk, diced; 2 Tbs. tomato paste; 1 Tbs. Cajun seasoning; 3 garlic cloves, minced; 3 cups (24 fl. oz./750 ml) chicken stock; 1 can (14 1/2 oz./455 g) diced tomatoes with juices; 2 tsp. filé powder; 1 lb. (500 g) medium shrimp, peeled and deveined, with tails intact; Steamed white rice for serving