Use the getFormat() method on the request object for content type negotiation,...

Use the getFormat() method on the request object for content type negotiation, and fallback to html in the case of no Accept header.

core/lib/Drupal/Core/ContentNegotiation.php

@@ -27,8 +27,6 @@ class ContentNegotiation {
    *   The request object from which to extract the content type.
    */
   public function getContentType(Request $request) {
-    $acceptable_content_types = $request->getAcceptableContentTypes();
-
     // AJAX iframe uploads need special handling, because they contain a json
     // response wrapped in <textarea>.
     if ($request->get('ajax_iframe_upload', FALSE)) {
@@ -40,16 +38,16 @@ public function getContentType(Request $request) {
       return 'ajax';
     }
 
-    // JSON requests can be responded to using JsonResponse().
-    elseif (in_array('application/json', $acceptable_content_types)) {
-      return 'json';
+    foreach ($request->getAcceptableContentTypes() as $mime_type) {
+      $format = $request->getFormat($mime_type);
+      if (!is_null($format)) {
+        return $format;
+      }
     }
 
-    // Do HTML last so that it always wins for */* formats.
-    elseif(in_array('text/html', $acceptable_content_types) || in_array('*/*', $acceptable_content_types)) {
+    // Do HTML last so that it always wins.
     return 'html';
   }
-  }
 
 }