diff --git b/lib/rules/node/file-extension-in-import.d.ts a/lib/rules/node/file-extension-in-import.d.ts
index 652b18d..7261252 100644
--- b/lib/rules/node/file-extension-in-import.d.ts
+++ a/lib/rules/node/file-extension-in-import.d.ts
@@ -5,7 +5,7 @@ import type { RuleConfig } from '../rule-config';
  */
 export interface FileExtensionInImportConfig {
   tryExtensions?: string[];
-  [k: string]: 'always' | 'never';
+  [ext: `.${string}`]: 'always' | 'never';
 }
 
 /**