diff -u b/core/modules/file/src/Tests/SaveUploadFormTest.php b/core/modules/file/src/Tests/SaveUploadFormTest.php
--- b/core/modules/file/src/Tests/SaveUploadFormTest.php
+++ b/core/modules/file/src/Tests/SaveUploadFormTest.php
@@ -312,6 +312,8 @@
    * Tests replacement when uploading over a file that already exists.
    */
   public function testExistingReplace() {
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
     $edit = [
       'file_test_replace' => FILE_EXISTS_REPLACE,
       'files[file_test_upload][]' => $file_system->realpath($this->image->getFileUri())