Issue #2587755 by aerozeppelin, NickWilde, swentel: AJAX error when using...

Issue #2587755 by aerozeppelin, NickWilde, swentel: AJAX error when using progress bar on file field widget

core/modules/file/file.routing.yml

 file.ajax_progress:
-  path: '/file/progress'
+  path: '/file/progress/{key}'
   defaults:
     _controller: '\Drupal\file\Controller\FileWidgetAjaxController::progress'
   requirements:

core/modules/file/src/Element/ManagedFile.php

@@ -271,7 +271,7 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
       }
 
       // Add the upload progress callback.
-      $element['upload_button']['#ajax']['progress']['url'] = Url::fromRoute('file.ajax_progress');
+      $element['upload_button']['#ajax']['progress']['url'] = Url::fromRoute('file.ajax_progress', ['key' => $upload_progress_key]);
     }
 
     // The file upload field itself.

core/modules/file/src/Tests/FileFieldWidgetTest.php

@@ -447,6 +447,12 @@ public function testWidgetElement() {
 
     // If the field has at least a item, the table should be visible.
     $this->assertIdentical(count($elements), 1);
+
+    // Test for AJAX error when using progress bar on file field widget
+    $key = $this->randomMachineName();
+    $this->drupalPost('file/progress/' . $key, 'application/json', []);
+    $this->assertNoResponse(500, t('No AJAX error when using progress bar on file field widget'));
+    $this->assertText('Starting upload...');
   }
 
 }