diff -u b/core/tests/Drupal/FunctionalJavascriptTests/DrupalSelenium2Driver.php b/core/tests/Drupal/FunctionalJavascriptTests/DrupalSelenium2Driver.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/DrupalSelenium2Driver.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/DrupalSelenium2Driver.php
@@ -3,8 +3,7 @@
 namespace Drupal\FunctionalJavascriptTests;
 
 use Behat\Mink\Driver\Selenium2Driver;
-use Behat\Mink\Exception\DriverException;
-use WebDriver\Exception\UnknownError;
+use Drupal\Tests\Traits\Core\SeleniumUploadRemoteFileTrait;
 use WebDriver\ServiceFactory;
 
 /**
@@ -12,6 +11,8 @@
  */
 class DrupalSelenium2Driver extends Selenium2Driver {
 
+  use SeleniumUploadRemoteFileTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -45,64 +46,2 @@
 
-  /**
-   * Uploads a file to the Selenium instance and returns the remote path.
-   *
-   * See \Behat\Mink\Driver\Selenium2Driver::uploadFile().
-   *
-   * @param string $path
-   *   The path to the file to upload.
-   *
-   * @return string
-   *   The remote path.
-   *
-   * @throws \Behat\Mink\Exception\DriverException
-   *   When PHP is compiled without zip support, or the file doesn't exist.
-   * @throws \WebDriver\Exception\UnknownError;
-   *   When an unknown error occurred during file upload.
-   * @throws \Exception
-   *   When a known error occurred during file upload.
-   */
-  public function getRemoteFilePath($path) {
-    if (!is_file($path)) {
-      throw new DriverException('File does not exist locally and cannot be uploaded to the remote instance.');
-    }
-
-    if (!class_exists('ZipArchive')) {
-      throw new DriverException('Could not compress file, PHP is compiled without zip support.');
-    }
-
-    // Selenium only accepts uploads that are compressed as a Zip archive.
-    $tempFilename = tempnam('', 'WebDriverZip');
-
-    $archive = new \ZipArchive();
-    $result = $archive->open($tempFilename, \ZipArchive::CREATE);
-    if (!$result) {
-      throw new DriverException('Zip archive could not be created. Error ' . $result);
-    }
-    $result = $archive->addFile($path, basename($path));
-    if (!$result) {
-      throw new DriverException('File could not be added to zip archive.');
-    }
-    $result = $archive->close();
-    if (!$result) {
-      throw new DriverException('Zip archive could not be closed.');
-    }
-
-    try {
-      $remotePath = $this->getWebDriverSession()->file(['file' => base64_encode(file_get_contents($tempFilename))]);
-
-      // If no path is returned the file upload failed silently.
-      if (empty($remotePath)) {
-        throw new UnknownError;
-      }
-    }
-    catch (\Exception $e) {
-      throw $e;
-    }
-    finally {
-      unlink($tempFilename);
-    }
-
-    return $remotePath;
-  }
-
 }
diff -u b/core/tests/Drupal/FunctionalJavascriptTests/Tests/DrupalSelenium2DriverTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Tests/DrupalSelenium2DriverTest.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/Tests/DrupalSelenium2DriverTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Tests/DrupalSelenium2DriverTest.php
@@ -27,27 +27,11 @@
   protected static $modules = ['file', 'field_ui', 'entity_test'];
 
   /**
-   * WebDriver.
-   *
-   * @var \Drupal\FunctionalJavascriptTests\DrupalSelenium2Driver
-   */
-  protected $webDriver;
-
-  /**
-   * The file system service.
-   *
-   * @var \Drupal\Core\File\FileSystem
-   */
-  protected $fileSystem;
-
-  /**
    * {@inheritdoc}
    */
   protected function setUp() {
     parent::setUp();
-    $this->webDriver = $this->getSession()->getDriver();
-    $this->fileSystem = \Drupal::service('file_system');
-    $storage_settings = ['cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED];
+    $storage_settings = ['cardinality' => 3];
     $this->createFileField('field_file', 'entity_test', 'entity_test', $storage_settings);
     $this->drupalLogin($this->drupalCreateUser([
       'administer entity_test content',
@@ -57,8 +41,6 @@
 
   /**
    * Tests uploading remote files.
-   *
-   * @covers ::getRemoteFilePath
    */
   public function testGetRemoteFilePath() {
     $entity = EntityTest::create();
@@ -67,14 +49,14 @@
     $files = array_slice($this->getTestFiles('text'), 0, 3);
     $real_paths = [];
     foreach ($files as $file) {
-      $real_paths[] = $this->fileSystem->realpath($file->uri);
+      $real_paths[] = \Drupal::service('file_system')->realpath($file->uri);
     }
     $remote_paths = [];
     foreach ($real_paths as $path) {
-      $remote_paths[] = $this->webDriver->getRemoteFilePath($path);
+      $remote_paths[] = $this->getSession()->getDriver()->uploadFileAndGetRemoteFilePath($path);
     }
 
-    // Tests that uploading remote files works with remote path.
+    // Tests that uploading multiple remote files works with remote path.
     $this->drupalGet($entity->toUrl('edit-form'));
     $multiple_field = $this->xpath('//input[@multiple]')[0];
     $multiple_field->setValue(implode("\n", $remote_paths));
@@ -82,23 +64,6 @@
     $this->getSession()->getPage()->findButton('Save')->click();
     $entity = EntityTest::load($entity->id());
     $this->assertCount(3, $entity->field_file);
-
-    // Tests that upload remote files doesn't works without remote path.
-    $entity = EntityTest::create();
-    $entity->save();
-    $this->drupalGet($entity->toUrl('edit-form'));
-    $multiple_field = $this->xpath('//input[@multiple]')[0];
-    try {
-      $multiple_field->setValue(implode("\n", $real_paths));
-      $this->assertSession()->assertWaitOnAjaxRequest();
-      $this->getSession()->getPage()->findButton('Save')->click();
-      $entity = EntityTest::load($entity->id());
-      $this->assertCount(0, $entity->field_file);
-      $this->fail('setValue() works with real path without problem! Can we remove our workaround with getRemoteFilePath() method?');
-    }
-    catch (\Exception $e) {
-      $this->assertContains('File not found', $e->getMessage());
-    }
   }
 
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/Tests/Traits/Core/SeleniumUploadRemoteFileTrait.php
@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\Tests\Traits\Core;
+
+use Behat\Mink\Exception\DriverException;
+use WebDriver\Exception\UnknownError;
+
+/**
+ * Adds the ability to upload a file to the Selenium instance.
+ */
+trait SeleniumUploadRemoteFileTrait {
+
+  /**
+   * Uploads a file to the Selenium instance and returns the remote path.
+   *
+   * \Behat\Mink\Driver\Selenium2Driver::uploadFile() is a private method so
+   * that can't be used inside a test, but we need the remote path that is
+   * generated when uploading to make sure the file reference exists on the
+   * container running selenium.
+   *
+   * @param string $path
+   *   The path to the file to upload.
+   *
+   * @return string
+   *   The remote path.
+   *
+   * @throws \Behat\Mink\Exception\DriverException
+   *   When PHP is compiled without zip support, or the file doesn't exist.
+   * @throws \WebDriver\Exception\UnknownError;
+   *   When an unknown error occurred during file upload.
+   * @throws \Exception
+   *   When a known error occurred during file upload.
+   */
+  public function uploadFileAndGetRemoteFilePath($path) {
+    if (!is_file($path)) {
+      throw new DriverException('File does not exist locally and cannot be uploaded to the remote instance.');
+    }
+
+    if (!class_exists('ZipArchive')) {
+      throw new DriverException('Could not compress file, PHP is compiled without zip support.');
+    }
+
+    // Selenium only accepts uploads that are compressed as a Zip archive.
+    $tempFilename = tempnam('', 'WebDriverZip');
+
+    $archive = new \ZipArchive();
+    $result = $archive->open($tempFilename, \ZipArchive::CREATE);
+    if (!$result) {
+      throw new DriverException('Zip archive could not be created. Error ' . $result);
+    }
+    $result = $archive->addFile($path, basename($path));
+    if (!$result) {
+      throw new DriverException('File could not be added to zip archive.');
+    }
+    $result = $archive->close();
+    if (!$result) {
+      throw new DriverException('Zip archive could not be closed.');
+    }
+
+    try {
+      $remotePath = $this->getWebDriverSession()->file(['file' => base64_encode(file_get_contents($tempFilename))]);
+
+      // If no path is returned the file upload failed silently.
+      if (empty($remotePath)) {
+        throw new UnknownError;
+      }
+    }
+    catch (\Exception $e) {
+      throw $e;
+    }
+    finally {
+      unlink($tempFilename);
+    }
+
+    return $remotePath;
+  }
+
+}