diff --git a/core/modules/file/src/Element/ManagedFile.php b/core/modules/file/src/Element/ManagedFile.php
index 8f2f1d2..07c2ef5 100644
--- a/core/modules/file/src/Element/ManagedFile.php
+++ b/core/modules/file/src/Element/ManagedFile.php
@@ -305,6 +305,10 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
       }
     }
 
+    if (isset($element['#upload_validators']['file_validate_extensions'][0])) {
+      $element['upload']['#attributes']['data-drupal-validate-extensions'] = $element['#upload_validators']['file_validate_extensions'][0];
+    }
+
     // Let #id point to the file element, so the field label's 'for' corresponds
     // with it.
     $element['#id'] = &$element['upload']['#id'];
diff --git a/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php b/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
index 16cdf83..1f11125 100644
--- a/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
+++ b/core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
@@ -236,7 +236,6 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
       '#type' => 'managed_file',
       '#upload_location' => $items[$delta]->getUploadLocation(),
       '#upload_validators' => $items[$delta]->getUploadValidators(),
-      '#after_build' => array(array($this, 'addValidateDataForJs')),
       '#value_callback' => array(get_class($this), 'value'),
       '#process' => array_merge($element_info['#process'], array(array(get_class($this), 'process'))),
       '#progress_indicator' => $this->getSetting('progress_indicator'),
@@ -279,26 +278,6 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
   }
 
   /**
-   * Form API callback. Attaches data- attributes to the file input field for
-   * JavaScript's benefit.
-   *
-   * @param array $element
-   *   The FileWidget's render array.
-   * @param FormStateInterface $form_state
-   *   The form state.
-   *
-   * @return array
-   *   The updated render array.
-   */
-  public function addValidateDataForJs(array $element, FormStateInterface &$form_state) {
-    // File extensions go on the ManagedFile's 'upload' child as data attribute.
-    if (is_array($element['upload']) && !empty($element['#upload_validators']['file_validate_extensions'])) {
-      $element['upload']['#attributes']['data-drupal-validate-extensions'] = $element['#upload_validators']['file_validate_extensions'][0];
-    }
-    return $element;
-  }
-
-  /**
    * {@inheritdoc}
    */
   public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
diff --git a/core/modules/file/src/Tests/FileManagedFileElementTest.php b/core/modules/file/src/Tests/FileManagedFileElementTest.php
index e1528bc..a58c846 100644
--- a/core/modules/file/src/Tests/FileManagedFileElementTest.php
+++ b/core/modules/file/src/Tests/FileManagedFileElementTest.php
@@ -23,6 +23,10 @@ function testManagedFile() {
     $this->drupalGet('file/test');
     $this->assertFieldByXpath('//input[@name="files[nested_file]" and @size="13"]', NULL, 'The custom #size attribute is passed to the child upload element.');
 
+    // Check that relevant data- attributes are present for validations
+    $this->drupalGet('file/test/validation/extension');
+    $this->assertFieldByXPath('//input[@name="files[file]" and @data-drupal-validate-extensions="txt"]', NULL, 'The data-drupal-validate-extensions attribute is present on the upload element.');
+
     // Perform the tests with all permutations of $form['#tree'],
     // $element['#extended'], and $element['#multiple'].
     $test_file = $this->getTestFile('text');
diff --git a/core/modules/file/tests/file_module_test/file_module_test.routing.yml b/core/modules/file/tests/file_module_test/file_module_test.routing.yml
index 15f7d0f..0c85f6e 100644
--- a/core/modules/file/tests/file_module_test/file_module_test.routing.yml
+++ b/core/modules/file/tests/file_module_test/file_module_test.routing.yml
@@ -8,3 +8,11 @@ file_module_test.managed_test:
     default_fids: NULL
   requirements:
     _access: 'TRUE'
+
+file_module_test.managed_validations_test:
+  path: '/file/test/validation/{validation_type}'
+  defaults:
+    _form: '\Drupal\file_module_test\Form\FileModuleTestValidationForm'
+    validation_type: 'extension'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/file/tests/file_module_test/src/Form/FileModuleTestValidationForm.php b/core/modules/file/tests/file_module_test/src/Form/FileModuleTestValidationForm.php
new file mode 100644
index 0000000..9674b09
--- /dev/null
+++ b/core/modules/file/tests/file_module_test/src/Form/FileModuleTestValidationForm.php
@@ -0,0 +1,70 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\file_module_test\Form\FileModuleTestForm.
+ */
+
+namespace Drupal\file_module_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Form controller for file_module_test module.
+ */
+class FileModuleTestValidationForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'file_module_test_validation_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param string $validation_type
+   *   The validation being tested, ie 'extension', 'size'
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, $validation_type = 'extension') {
+    switch ($validation_type) {
+      case 'extension':
+        $validator = ['file_validate_extensions' => ['txt']];
+        break;
+      case 'size':
+        $validator = ['file_validate_size' => [1024]];
+        break;
+      default:
+        return FALSE;
+    }
+
+    $form['file'] = array(
+      '#type' => 'managed_file',
+      '#title' => $this->t('Managed File'),
+      '#upload_location' => 'public://test',
+      '#progress_message' => $this->t('Please wait...'),
+      '#upload_validators' => $validator,
+    );
+
+    $form['submit'] = array(
+      '#type' => 'submit',
+      '#value' => $this->t('Save'),
+    );
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    drupal_set_message($this->t('Test submission handler was allowed to execute.'));
+  }
+
+}