diff --git a/packages/typescript-estree/src/create-program/WatchCompilerHostOfConfigFile.ts b/packages/typescript-estree/src/create-program/WatchCompilerHostOfConfigFile.ts
index 471a6d6b6fe..565f58417c8 100644
--- a/packages/typescript-estree/src/create-program/WatchCompilerHostOfConfigFile.ts
+++ b/packages/typescript-estree/src/create-program/WatchCompilerHostOfConfigFile.ts
@@ -32,6 +32,7 @@ interface WatchCompilerHostOfConfigFile<T extends ts.BuilderProgram>
   onCachedDirectoryStructureHostCreate(
     host: CachedDirectoryStructureHost,
   ): void;
+  extraFileExtensions?: readonly ts.FileExtensionInfo[];
 }
 
 export { WatchCompilerHostOfConfigFile };
diff --git a/packages/typescript-estree/src/create-program/createWatchProgram.ts b/packages/typescript-estree/src/create-program/createWatchProgram.ts
index 682e1d7c731..cf31a5c383d 100644
--- a/packages/typescript-estree/src/create-program/createWatchProgram.ts
+++ b/packages/typescript-estree/src/create-program/createWatchProgram.ts
@@ -314,6 +314,14 @@ function createWatchProgram(
       );
     oldOnDirectoryStructureHostCreate(host);
   };
+  // This works only on 3.9
+  watchCompilerHost.extraFileExtensions = extra.extraFileExtensions.map(
+    extension => ({
+      extension,
+      isMixedContent: true,
+      scriptKind: ts.ScriptKind.Deferred,
+    }),
+  );
   watchCompilerHost.trace = log;
 
   // Since we don't want to asynchronously update program we want to disable timeout methods