diff --git a/core/lib/Drupal/Core/FormatMapping.php b/core/lib/Drupal/Core/FormatMapping.php
index 3b3166c..40e6aea 100644
--- a/core/lib/Drupal/Core/FormatMapping.php
+++ b/core/lib/Drupal/Core/FormatMapping.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\Core\FormatMapping.
+ * Contains \Drupal\Core\FormatMapping.
  */
 
 namespace Drupal\Core;
@@ -36,7 +36,7 @@ public function getContentType(Request $request) {
       return 'iframeupload';
     }
 
-    if ($type = $request->query->get('dataType')) {
+    if ($type = $request->query->get('dialogType')) {
       return 'drupal_' . $type;
     }
 
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index ba47f97..87b9f02 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -240,6 +240,7 @@ public static function preRenderAjaxForm($element) {
       $settings += array(
         'url' => isset($settings['callback']) ? Url::fromRoute('system.ajax') : NULL,
         'options' => array(),
+        'dialogType' => 'ajax',
       );
 
       // @todo Legacy support. Remove in Drupal 8.
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index c788c16..9bd28f1 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -255,7 +255,7 @@
       ajax.options.data.dialogOptions = element_settings.dialog;
     }
     if (element_settings.dialogType) {
-      ajax.options.url += '?dataType=' + element_settings.dialogType;
+      ajax.options.url += '?dialogType=' + element_settings.dialogType;
     }
 
     // Bind the ajaxSubmit function to the element event.
diff --git a/core/modules/system/src/Tests/Ajax/DialogTest.php b/core/modules/system/src/Tests/Ajax/DialogTest.php
index 482e025..28a3d0c 100644
--- a/core/modules/system/src/Tests/Ajax/DialogTest.php
+++ b/core/modules/system/src/Tests/Ajax/DialogTest.php
@@ -165,7 +165,7 @@ public function testDialog() {
         'callback' => '::preview',
         'event' => 'click',
         'url' => Url::fromRoute('system.ajax')->toString(),
-        'accepts' => 'application/vnd.drupal-ajax',
+        'dialogType' => 'ajax',
         'submit' => [
           '_triggering_element_name' => 'op',
           '_triggering_element_value' => 'Preview',
diff --git a/core/tests/Drupal/Tests/Core/FormatMappingTest.php b/core/tests/Drupal/Tests/Core/FormatMappingTest.php
index 36921e8..f24e09b 100644
--- a/core/tests/Drupal/Tests/Core/FormatMappingTest.php
+++ b/core/tests/Drupal/Tests/Core/FormatMappingTest.php
@@ -15,12 +15,12 @@
  * @coversDefaultClass \Drupal\Core\FormatMapping
  * @group FormatMapping
  */
-class ContentNegotiationTest extends UnitTestCase {
+class FormatMappingTest extends UnitTestCase {
 
   /**
    * @var \Drupal\Core\FormatMapping
    */
-  protected $contentNegotiation;
+  protected $formatMapping;
 
   /**
    * {@inheritdoc}
@@ -28,7 +28,7 @@ class ContentNegotiationTest extends UnitTestCase {
   protected function setUp() {
     parent::setUp();
 
-    $this->contentNegotiation = new FormatMapping;
+    $this->formatMapping = new FormatMapping;
   }
 
   /**
@@ -40,7 +40,7 @@ public function testAjaxIframeUpload() {
     $request = new Request();
     $request->attributes->set('ajax_iframe_upload', '1');
 
-    $this->assertSame('iframeupload', $this->contentNegotiation->getContentType($request));
+    $this->assertSame('iframeupload', $this->formatMapping->getContentType($request));
   }
 
   /**
@@ -51,19 +51,21 @@ public function testAjaxIframeUpload() {
    */
   public function testAPriorityFormatIsFound($priority, $format) {
     $request = new Request();
-    $request->setFormat($format['format'], $format['mime_type']);
-    $request->headers->set('Accept', sprintf('%s,application/json', $format['mime_type']));
 
-    $this->assertSame($priority, $this->contentNegotiation->getContentType($request));
+    if ($format) {
+      $request->query->set('dialogType', $format);
+    }
+
+    $this->assertSame($priority, $this->formatMapping->getContentType($request));
   }
 
   public function priorityFormatProvider()
   {
     return [
-      ['drupal_dialog', ['format' => 'drupal_dialog', 'mime_type' => 'application/vnd.drupal-dialog']],
-      ['drupal_modal', ['format' => 'drupal_modal', 'mime_type' => 'application/vnd.drupal-modal']],
-      ['drupal_ajax', ['format' => 'drupal_ajax', 'mime_type' => 'application/vnd.drupal-ajax']],
-      ['html', ['format' => 'html', 'mime_type' => 'text/html']],
+      ['drupal_dialog', 'dialog'],
+      ['drupal_modal', 'modal'],
+      ['drupal_ajax', 'ajax'],
+      ['html', ''],
     ];
   }
 
@@ -76,7 +78,7 @@ public function testNoPriorityFormatIsFoundButReturnsTheFirstValidOne() {
     $request = new Request();
     $request->headers->set('Accept', 'application/rdf+xml');
 
-    $this->assertSame('rdf', $this->contentNegotiation->getContentType($request));
+    $this->assertSame('rdf', $this->formatMapping->getContentType($request));
   }
 
   /**
@@ -87,7 +89,7 @@ public function testNoPriorityFormatIsFoundButReturnsTheFirstValidOne() {
   public function testUnknowContentTypeReturnsHtmlByDefault() {
     $request = new Request();
 
-    $this->assertSame('html', $this->contentNegotiation->getContentType($request));
+    $this->assertSame('html', $this->formatMapping->getContentType($request));
   }
 
   /**
@@ -99,7 +101,7 @@ public function testUnknowContentTypeButAjaxRequest() {
     $request = new Request();
     $request->headers->set('X-Requested-With', 'XMLHttpRequest');
 
-    $this->assertSame('ajax', $this->contentNegotiation->getContentType($request));
+    $this->assertSame('ajax', $this->formatMapping->getContentType($request));
   }
 
 }