Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/lib/Drupal/Core/ContentNegotiation.php

@@ -0,0 +1,47 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\ContentNegotiation.
+ */
+
+namespace Drupal\Core;
+
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Provides content negotation based upon query parameters.
+ */
+class ContentNegotiation {
+
+  /**
+   * Gets the normalized type of a request.
+   *
+   * The normalized type is a short, lowercase version of the format, such as
+   * 'html', 'json' or 'atom'.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request object from which to extract the content type.
+   *
+   * @return string
+   *   The normalized type of a given request.
+   */
+  public function getContentType(Request $request) {
+    // AJAX iframe uploads need special handling, because they contain a JSON
+    // response wrapped in <textarea>.
+    if ($request->get('ajax_iframe_upload', FALSE)) {
+      return 'iframeupload';
+    }
+
+    if ($request->query->has('_format')) {
+      return $request->query->get('_format');
+    }
+
+    if ($request->isXmlHttpRequest()) {
+      return 'ajax';
+    }
+
+    // Do HTML last so that it always wins.
+    return 'html';
+  }
+}