add file preview

Reviewed-on: sites/file-explorer#6

src/Forms/UploadFileForm.php

@@ -17,7 +17,7 @@ class UploadFileForm extends AbstractType
     {
         $builder
             ->setAction($this->urlGenerator->generate('app_upload'))
-            ->add('file', FileType::class, [
+            ->add('files', FileType::class, [
             'attr' => ['class' => 'hidden'],
             'multiple' => true,
         ]);