#3368149 MR for the changes.

src/Controller/FileController.php

@@ -7,11 +7,12 @@ use Drupal\Core\Ajax\CloseDialogCommand;
 use Drupal\Core\Ajax\OpenDialogCommand;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Form\FormState;
+use Drupal\Core\File\FileSystemInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\file\FileInterface;
 use Symfony\Component\HttpFoundation\BinaryFileResponse;
 use Symfony\Component\HttpFoundation\File\Exception\FileNotFoundException;
 use Symfony\Component\HttpFoundation\Response;
-use Drupal\Core\File\FileSystemInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -19,6 +20,18 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  */
 class FileController extends ControllerBase {
 
+  use StringTranslationTrait;
+
+  /**
+   * Constructs a new \Drupal\file_entity\Controller\FileController object.
+   *
+   * @param \Drupal\Core\StringTranslation\TranslationInterface $translation
+   *   The translation service.
+   */
+  public function __construct(TranslationInterface $translation) {
+    $this->setStringTranslation($translation);
+  }
+
   /**
    * The file system service.
    *
@@ -78,7 +91,7 @@ class FileController extends ControllerBase {
     // Ensure there is a valid token to download this file.
     if (!$this->config('file_entity.settings')->get('allow_insecure_download')) {
       if (!isset($_GET['token']) || $_GET['token'] !== $file->getDownloadToken()) {
-        return new Response(t('Access to file @url denied', array('@url' => $file->getFileUri())), 403);
+        return new Response($this->t('Access to file @url denied', array('@url' => $file->getFileUri())), 403);
       }
     }
 
@@ -99,7 +112,7 @@ class FileController extends ControllerBase {
       return new BinaryFileResponse($file->getFileUri(), 200, $headers, FALSE, 'attachment');
     }
     catch (FileNotFoundException $e) {
-      return new Response(t('File @uri not found', array('@uri' =>$file->getFileUri())), 404);
+      return new Response($this->t('File @uri not found', array('@uri' =>$file->getFileUri())), 404);
     }
   }
 

[Alberto Paderno]

The documentation comment for constructors is not mandatory anymore, If it is given, the description must be Constructs a new [class name] object. where [class name] includes the class namespace.