Prevent pirates from transforming folders

Author: Mark1626

packages/jest-transform/src/ScriptTransformer.ts

@@ -465,6 +465,7 @@ export default class ScriptTransformer {
     this.preloadTransformer(moduleName);
 
     let transforming = false;
+    const ext = path.extname(moduleName);
     const revertHook = addHook(
       (code, filename) => {
         try {
@@ -475,7 +476,7 @@ export default class ScriptTransformer {
         }
       },
       {
-        exts: [path.extname(moduleName)],
+        exts: ext === '' ? ['.js'] : [ext],
         ignoreNodeModules: false,
         matcher: filename => {
           if (transforming) {