composer update

web/core/modules/system/tests/themes/test_theme/test_theme.info.yml

@@ -16,7 +16,7 @@ base theme: classy
 core: 8.x
 logo: images/logo2.svg
 stylesheets-remove:
-  - '@system/css/js.module.css'
+  - '@stable/css/system/components/js.module.css'
 libraries:
   - test_theme/global-styling
 libraries-override:

web/core/modules/system/tests/themes/test_theme_settings/config/schema/test_theme_settings.schema.yml

@@ -10,3 +10,9 @@ test_theme_settings.settings:
       sequence:
         type: integer
         label: 'fids'
+    multi_file:
+      type: sequence
+      label: 'Multiple file field with all file extensions'
+      sequence:
+        type: integer
+        label: 'fids'

web/core/modules/system/tests/themes/test_theme_settings/theme-settings.php

@@ -24,6 +24,17 @@ function test_theme_settings_form_system_theme_settings_alter(&$form, FormStateI
     ],
   ];
 
+  $form['multi_file'] = [
+    '#type' => 'managed_file',
+    '#title' => t('Multiple file field with all file extensions'),
+    '#multiple' => TRUE,
+    '#default_value' => theme_get_setting('multi_file'),
+    '#upload_location' => 'public://test',
+    '#upload_validators'  => [
+      'file_validate_extensions' => [],
+    ],
+  ];
+
   $form['#submit'][] = 'test_theme_settings_form_system_theme_settings_submit';
 }