Make FileReferenceFieldItemListProcessor work.

src/Processor/FileReferenceFieldItemListProcessor.php

@@ -5,7 +5,7 @@ namespace Drupal\custom_elements\Processor;
 
 use Drupal\Core\Field\FieldItemListInterface;
 use Drupal\custom_elements\CustomElement;
-use Drupal\multiversion\FileItem;
+use Drupal\file\Plugin\Field\FieldType\FileItem;
 
 /**
  * Renders file references (including images) into single slots.
@@ -16,7 +16,7 @@ class FileReferenceFieldItemListProcessor implements CustomElementProcessorInter
    * {@inheritdoc}
    */
   public function supports($data, $viewMode) {
-    return $data instanceof FieldItemListInterface && $data->getFieldDefinition()->getType() == 'entity_reference' && $data->first() instanceof FileItem;
+    return $data instanceof FieldItemListInterface && $data->first() instanceof FileItem;
   }
 
   /**