diff --git a/core/core.services.yml b/core/core.services.yml
index 6127c8f..3bae441 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -568,7 +568,7 @@ services:
     class: Symfony\Component\HttpKernel\HttpKernel
     arguments: ['@event_dispatcher', '@controller_resolver', '@request_stack']
   http_negotiation.format_negotiator:
-    class: Drupal\Core\ContentNegotiation
+    class: Drupal\Core\FormatMapping
     private: true
   http_middleware.negotiation:
     class: Drupal\Core\StackMiddleware\NegotiationMiddleware
diff --git a/core/lib/Drupal/Core/FormatMapping.php b/core/lib/Drupal/Core/FormatMapping.php
index 1248a69..3b3166c 100644
--- a/core/lib/Drupal/Core/FormatMapping.php
+++ b/core/lib/Drupal/Core/FormatMapping.php
@@ -36,12 +36,15 @@ public function getContentType(Request $request) {
       return 'iframeupload';
     }
 
+    if ($type = $request->query->get('dataType')) {
+      return 'drupal_' . $type;
+    }
+
     // Check all formats, if priority format is found return it.
     $first_found_format = FALSE;
-    $priority = array('html', 'drupal_ajax', 'drupal_modal', 'drupal_dialog');
     foreach ($request->getAcceptableContentTypes() as $mime_type) {
       $format = $request->getFormat($mime_type);
-      if (in_array($format, $priority, TRUE)) {
+      if ($format === 'html') {
         return $format;
       }
       if (!is_null($format) && !$first_found_format) {
diff --git a/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php b/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
index 295857d..8fa0173 100644
--- a/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
+++ b/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\StackMiddleware;
 
-use Drupal\Core\ContentNegotiation;
+use Drupal\Core\FormatMapping;
 use Drupal\Core\ContentNegotiationInterface;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
@@ -29,7 +29,7 @@ class NegotiationMiddleware implements HttpKernelInterface {
   /**
    * The content negotiator.
    *
-   * @var \Drupal\Core\ContentNegotiation
+   * @var \Drupal\Core\FormatMapping
    */
   protected $negotiator;
 
@@ -45,10 +45,10 @@ class NegotiationMiddleware implements HttpKernelInterface {
    *
    * @param \Symfony\Component\HttpKernel\HttpKernelInterface $app
    *   The wrapper HTTP kernel
-   * @param \Drupal\Core\ContentNegotiation $negotiator
+   * @param \Drupal\Core\FormatMapping $negotiator
    *   The content negotiator.
    */
-  public function __construct(HttpKernelInterface $app, ContentNegotiation $negotiator) {
+  public function __construct(HttpKernelInterface $app, FormatMapping $negotiator) {
     $this->app = $app;
     $this->negotiator = $negotiator;
   }
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 5b64318..c788c16 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.data.dialogType = element_settings.dialogType;
+      ajax.options.url += '?dataType=' + element_settings.dialogType;
     }
 
     // Bind the ajaxSubmit function to the element event.
diff --git a/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php b/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php
index e411008..36921e8 100644
--- a/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php
+++ b/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php
@@ -7,18 +7,18 @@
 
 namespace Drupal\Tests\Core;
 
-use Drupal\Core\ContentNegotiation;
+use Drupal\Core\FormatMapping;
 use Drupal\Tests\UnitTestCase;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
- * @coversDefaultClass \Drupal\Core\ContentNegotiation
- * @group ContentNegotiation
+ * @coversDefaultClass \Drupal\Core\FormatMapping
+ * @group FormatMapping
  */
 class ContentNegotiationTest extends UnitTestCase {
 
   /**
-   * @var \Drupal\Core\ContentNegotiation
+   * @var \Drupal\Core\FormatMapping
    */
   protected $contentNegotiation;
 
@@ -28,7 +28,7 @@ class ContentNegotiationTest extends UnitTestCase {
   protected function setUp() {
     parent::setUp();
 
-    $this->contentNegotiation = new ContentNegotiation;
+    $this->contentNegotiation = new FormatMapping;
   }
 
   /**