diff -u b/core/modules/file/file.module b/core/modules/file/file.module
--- b/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -702,10 +702,11 @@
  */
 function file_save_upload_from_form($element, FormStateInterface $form_state, $delta = NULL, $replace = FILE_EXISTS_RENAME) {
   $destination = isset($element['#upload_location']) ? $element['#upload_location'] : FALSE;
+  $validators = isset($element['#upload_validators']) ? $element['#upload_validators'] : array();
   $upload_name = implode('_', $element['#parents']);
   $errors_before = drupal_get_messages('error');
 
-  $files = file_save_upload($upload_name, $element['#upload_validators'], $destination, $delta, $replace);
+  $files = file_save_upload($upload_name, $validators, $destination, $delta, $replace);
 
   // Get all possible new errors that are generated while trying to save the
   // upload.
@@ -835,7 +836,7 @@
           break;
         }
 
-        // Unknown error
+      // Unknown error
       default:
         drupal_set_message(t('The file %file could not be saved. An unknown error has occurred.', array('%file' => $file_info->getFilename())), 'error');
         $files[$i] = FALSE;
@@ -1279,7 +1280,6 @@
 
   return array();
 }
-
 /**
  * Prepares variables for file form widget templates.
  *
diff -u b/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
--- b/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -357,9 +357,10 @@
     if ($this->moduleHandler->moduleExists('file')) {
       // Handle file uploads.
       $validators = array('file_validate_is_image' => array());
+      $logo_upload_element = $form['logo']['settings']['logo_upload'];
 
       // Check for a new uploaded logo.
-      $file = file_save_upload_from_form($form['logo_upload'], $form_state, 0);
+      $file = file_save_upload_from_form($logo_upload_element, $form_state, 0);
       if (isset($file)) {
         // File upload was attempted.
         if ($file) {
@@ -368,14 +369,14 @@
         }
         else {
           // File upload failed.
-          $form_state->setErrorByName('logo_upload', $this->t('The logo could not be uploaded.'));
+          $form_state->setErrorByName($logo_upload_element, $this->t('The logo could not be uploaded.'));
         }
       }
 
       $validators = array('file_validate_extensions' => array('ico png gif jpg jpeg apng svg'));
 
       // Check for a new uploaded favicon.
-      $file = file_save_upload_from_form($form['favicon_upload'], $form_state, 0);
+      $file = file_save_upload_from_form($form['favicon']['settings']['favicon_upload'], $form_state, 0);
       if (isset($file)) {
         // File upload was attempted.
         if ($file) {