diff --git a/core/lib/Drupal/Core/Render/Element/File.php b/core/lib/Drupal/Core/Render/Element/File.php
index b8d5263..e18564d 100644
--- a/core/lib/Drupal/Core/Render/Element/File.php
+++ b/core/lib/Drupal/Core/Render/Element/File.php
@@ -49,7 +49,7 @@ public function getInfo() {
    */
   public static function processFile(&$element, FormStateInterface $form_state, &$complete_form) {
     if ($element['#multiple']) {
-      $element['#attributes'] = array('multiple' => 'multiple');
+      $element['#attributes'] += array('multiple' => 'multiple');
       $element['#name'] .= '[]';
     }
     return $element;
diff --git a/core/modules/file/file.js b/core/modules/file/file.js
index 5a4aea8..170afd0 100644
--- a/core/modules/file/file.js
+++ b/core/modules/file/file.js
@@ -17,35 +17,21 @@
    * @type {Drupal~behavior}
    */
   Drupal.behaviors.fileValidateAutoAttach = {
-    attach: function (context, settings) {
+    attach: function (context) {
       var $context = $(context);
-      var elements;
-
-      function initFileValidation(selector) {
-        $context.find(selector)
-          .once('fileValidate')
-          .on('change.fileValidate', {extensions: elements[selector]}, Drupal.file.validateExtension);
-      }
 
-      if (settings.file && settings.file.elements) {
-        elements = settings.file.elements;
-        Object.keys(elements).forEach(initFileValidation);
-      }
+      $context.find('.js-form-managed-file .js-form-file')
+        .once('fileValidate')
+        .on('change.fileValidate', Drupal.file.validateExtension);
     },
     detach: function (context, settings, trigger) {
       var $context = $(context);
-      var elements;
 
-      function removeFileValidation(selector) {
-        $context.find(selector)
+      if (trigger === 'unload') {
+        $context.find('.js-form-managed-file .js-form-file')
           .removeOnce('fileValidate')
           .off('change.fileValidate', Drupal.file.validateExtension);
       }
-
-      if (trigger === 'unload' && settings.file && settings.file.elements) {
-        elements = settings.file.elements;
-        Object.keys(elements).forEach(removeFileValidation);
-      }
     }
   };
 
@@ -117,10 +103,10 @@
       $('.file-upload-js-error').remove();
 
       // Add client side validation for the input[type=file].
-      var extensionPattern = event.data.extensions.replace(/,\s*/g, '|');
-      if (extensionPattern.length > 1 && this.value.length > 0) {
+      var extensionPattern = $(event.target).attr('data-drupal-validate-extensions').replace(/,|\s+/g, '|');
+      if (extensionPattern.length > 1 && event.target.value.length > 0) {
         var acceptableMatch = new RegExp('\\.(' + extensionPattern + ')$', 'gi');
-        if (!acceptableMatch.test(this.value)) {
+        if (!acceptableMatch.test(event.target.value)) {
           var error = Drupal.t("The selected file %filename cannot be uploaded. Only files with the following extensions are allowed: %extensions.", {
             // According to the specifications of HTML5, a file upload control
             // should not reveal the real local path to the file that a user
@@ -129,7 +115,7 @@
             // confusion by leaving the user thinking perhaps Drupal could not
             // find the file because it messed up the file path. To avoid this
             // confusion, therefore, we strip out the bogus fakepath string.
-            '%filename': this.value.replace('C:\\fakepath\\', ''),
+            '%filename': event.target.value.replace('C:\\fakepath\\', ''),
             '%extensions': extensionPattern.replace(/\|/g, ', ')
           });
           $(this).closest('div.js-form-managed-file').prepend('<div class="messages messages--error file-upload-js-error" aria-live="polite">' + error + '</div>');
diff --git a/core/modules/file/src/Element/ManagedFile.php b/core/modules/file/src/Element/ManagedFile.php
index c2b09d8..4ced11c 100644
--- a/core/modules/file/src/Element/ManagedFile.php
+++ b/core/modules/file/src/Element/ManagedFile.php
@@ -275,9 +275,15 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
     }
 
     // The file upload field itself.
+    $validate_attrs = [];
+    if(!empty($element['#upload_validators']['file_validate_extensions'])) {
+      $validate_attrs += ['data-drupal-validate-extensions' => implode(' ', $element['#upload_validators']['file_validate_extensions'])];
+    }
+
     $element['upload'] = [
       '#name' => 'files[' . $parents_prefix . ']',
       '#type' => 'file',
+      '#attributes' => $validate_attrs,
       '#title' => t('Choose a file'),
       '#title_display' => 'invisible',
       '#size' => $element['#size'],
@@ -305,12 +311,6 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
       }
     }
 
-    // Add the extension list to the page as JavaScript settings.
-    if (isset($element['#upload_validators']['file_validate_extensions'][0])) {
-      $extension_list = implode(',', array_filter(explode(' ', $element['#upload_validators']['file_validate_extensions'][0])));
-      $element['upload']['#attached']['drupalSettings']['file']['elements']['#' . $element['#id']] = $extension_list;
-    }
-
     // Let #for reference the file element by its array index. This causes
     // the label for the overall manged file form element to reference the
     // nested file input, see form.inc's template_preprocess_form_element.