diff -u b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
--- b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
@@ -27,7 +27,7 @@
     $request->setFormat('drupal_ajax', 'application/vnd.drupal-ajax');
     $request->setFormat('drupal_dialog', 'application/vnd.drupal-dialog');
     $request->setFormat('drupal_modal', 'application/vnd.drupal-modal');
-    $request->setFormat('iframeupload', 'application/vnd.drupal-ajax');
+    $request->setFormat('iframeupload', 'application/vnd.drupal-ie-lte-9-sucks-beyond-compare');
   }
 
   /**
only in patch2:
unchanged:
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -736,6 +736,11 @@ services:
     arguments: ['@title_resolver']
     tags:
       - { name: render.main_content_renderer, format: drupal_modal }
+  main_content_renderer.iframeupload:
+    class: Drupal\Core\Render\MainContent\AjaxRenderer
+    arguments: ['@title_resolver']
+    tags:
+      - { name: render.main_content_renderer, format: iframeupload }
   controller.form:
     class: Drupal\Core\Controller\HtmlFormController
     arguments: ['@controller_resolver', '@form_builder', '@class_resolver']