diff --git a/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php b/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
index cb55518..01723b2 100644
--- a/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
+++ b/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php
@@ -82,7 +82,7 @@ public function registerFormat($format, $mime_type) {
    * @return string
    *   The normalized type of a given request.
    */
-  public function getContentType(Request $request) {
+  private 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)) {