diff --git a/core/modules/file/tests/src/FunctionalJavascript/ClientValidationTest.php b/core/modules/file/tests/src/FunctionalJavascript/ClientValidationTest.php
new file mode 100644
index 0000000..b4f5593
--- /dev/null
+++ b/core/modules/file/tests/src/FunctionalJavascript/ClientValidationTest.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\Tests\file\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests the client side JavaScript validation.
+ *
+ * @group file
+ */
+class ClientValidationTest extends JavascriptTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['file', 'node'];
+
+  /**
+   * Tests the client side validation on file fields.
+   */
+  public function testDisallowedExtensionErrorMessage() {
+    $admin_user = $this->drupalCreateUser([
+      'create article content',
+      'administer site configuration',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    $this->drupalGet('node/add/article');
+    $page = $this->getSession()->getPage();
+
+    // Test uploading a file with a file extension not allowed.
+    // Try to upload a file with an unallowed extension
+
+    // @TODO http://phantomjs.org/api/webpage/method/upload-file.html
+    // page.uploadFile('input[name=image]', '/path/to/some/photo.jpg');
+
+    // Verify that the error message is there.
+    $error_selector = '.file-upload-js-error';
+    $assert = $this->assertSession();
+    $assert->elementExists('css', $error_selector);
+    $assert->elementContains('css', $error_selector, 'The selected file robots.txt cannot be uploaded. Only files with the following extensions are allowed: png, gif, jpg, jpeg.');
+    // Verify that there is only one error message on the page.
+    $condition = "(jQuery('$error_selector').length == 1)";
+    $assert->assertJsCondition($condition);
+  }
+
+}