reverted:
--- b/core/core.services.yml
+++ a/core/core.services.yml
@@ -736,11 +736,6 @@
     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']
diff -u b/core/lib/Drupal/Core/Ajax/AjaxResponse.php b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
--- b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
@@ -79,16 +79,29 @@
       $this->setData($this->ajaxRender($request));
     }
 
-    // Add proper HTTP Header for IE.
+    // IE 9 does not support XHR 2 (http://caniuse.com/#feat=xhr2), so
+    // for that browser, jquery.form submits requests containing a file upload
+    // via an IFRAME rather than via XHR. Since the response is being sent to
+    // an IFRAME, it must be formatted as HTML. Specifically:
+    // - It must use the text/html content type or else the browser will
+    //   present a download prompt.
+    // - It must place the JSON data into a textarea to prevent browser
+    //   extensions such as Linkification and Skype's Browser Highlighter
+    //   from applying HTML transformations such as URL or phone number to
+    //   link conversions on the data values.
+    //
+    // Since this affects the format of the output, it could be argued that
+    // this should be implemented as a separate Accept MIME type. However,
+    // that would require separate variants for each type of AJAX request
+    // (e.g., drupal-ajax, drupal-dialog, drupal-modal), so for expediency,
+    // this browser workaround is implemented via a GET or POST parameter.
+    //
+    // @see http://malsup.com/jquery/form/#file-upload
+    // @see https://drupal.org/node/1009382
+    // @see https://drupal.org/node/2339491
+    // @see Drupal.ajax.prototype.beforeSend()
     if ($request->get('ajax_iframe_upload', FALSE)) {
       $this->headers->set('Content-Type', 'text/html; charset=utf-8');
-
-      // Browser IFRAMEs expect HTML. Browser extensions, such as Linkification
-      // and Skype's Browser Highlighter, convert URLs, phone numbers, etc. into
-      // links. This corrupts the JSON response. Protect the integrity of the
-      // JSON data by making it the value of a textarea.
-      // @see http://malsup.com/jquery/form/#file-upload
-      // @see http://drupal.org/node/1009382
       $this->setContent('<textarea>' . $this->data  . '</textarea>');
     }
   }
reverted:
--- b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
+++ a/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
@@ -27,7 +27,6 @@
     $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-ie-lte-9-sucks-beyond-compare');
   }
 
   /**
diff -u b/core/lib/Drupal/Core/ContentNegotiation.php b/core/lib/Drupal/Core/ContentNegotiation.php
--- b/core/lib/Drupal/Core/ContentNegotiation.php
+++ b/core/lib/Drupal/Core/ContentNegotiation.php
@@ -33,7 +33,7 @@
 
     // Check all formats, if priority format is found return it.
     $first_found_format = FALSE;
-    $priority = array('html', 'drupal_ajax', 'drupal_modal', 'drupal_dialog', 'iframeupload');
+    $priority = array('html', 'drupal_ajax', 'drupal_modal', 'drupal_dialog');
     foreach ($request->getAcceptableContentTypes() as $mime_type) {
       $format = $request->getFormat($mime_type);
       if (in_array($format, $priority, TRUE)) {
reverted:
--- b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -209,7 +209,7 @@
 
     // These are all JSON errors for our purposes. Any special handling for
     // them can/should happen in earlier listeners if desired.
+    if (in_array($format, ['drupal_modal', 'drupal_dialog', 'drupal_ajax'])) {
-    if (in_array($format, ['drupal_modal', 'drupal_dialog', 'drupal_ajax', 'iframeupload'])) {
       $format = 'json';
     }