Issue #3455468: serve as plain/text

Closes #3455468

src/Controller/ApplePayVerificationController.php

@@ -60,6 +60,7 @@ final class ApplePayVerificationController extends ControllerBase {
     $file = $this->entityTypeManager->getStorage('file')->load($fileId);
 
     $response = new CacheableResponse(file_get_contents($file->getFileUri()));
+    $response->headers->set('Content-Type', 'text/plain');
     return $response;
   }