diff --git a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
index 22e2abfb4f..f84ae1d2ef 100644
--- a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
+++ b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
@@ -51,7 +51,7 @@ class FileUploadResource extends ResourceBase {
    *
    * @var string
    */
-  const FILENAME_REGEX = '@\bfilename=\"(?<filename>.+)\"@';
+  const FILENAME_REGEX = '@\bfilename(?<star>\*?)=\"(?<filename>.+)\"@';
 
   /**
    * @var \Drupal\Core\File\FileSystem
@@ -192,7 +192,7 @@ public function post(Request $request, $entity_type_id, $bundle, $field_name) {
     $lock_id = $this->generateLockIdFromFileUri($file_uri);
 
     if (!$this->lock->acquire($lock_id)) {
-      throw new HttpException(500, sprintf('File "%s" is already locked for writing'));
+      throw new HttpException(503, sprintf('File "%s" is already locked for writing'), NULL, ['Retry-After' => 1]);
     }
 
     // Begin building file entity.
diff --git a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
index 920f3e78b2..cc42a53d19 100644
--- a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
@@ -233,6 +233,30 @@ public function testFileUploadStrippedFilePath() {
     $this->assertSame($this->testFileData, file_get_contents('public://foobar/example.txt'));
   }
 
+  /**
+   * Tests using the file upload route with a unicode file name.
+   */
+  public function testFileUploadUnicodeFilename() {
+    $this->initAuthentication();
+
+    $this->provisionResource([static::$format], static::$auth ? [static::$auth] : [], ['POST']);
+
+    $this->setUpAuthorization('POST');
+
+    $uri = Url::fromUri('base:' . static::$postUri);
+
+    $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => 'file; filename="example-✓.txt"']);
+
+    $this->assertSame(201, $response->getStatusCode());
+
+    $expected = $this->getExpectedNormalizedEntity(1, 'example-✓.txt', TRUE);
+    $this->assertResponseData($expected, $response);
+
+    // Check the actual file data. It should have been written to the configured
+    // directory, not /foobar/directory/example.txt.
+    $this->assertSame($this->testFileData, file_get_contents('public://foobar/example-✓.txt'));
+  }
+
   /**
    * Tests using the file upload route with a zero byte file.
    */
@@ -330,11 +354,13 @@ protected function getExpectedUnauthorizedAccessMessage($method) {
    *   The file ID to load and create normalized data for.
    * @param string $expected_filename
    *   The expected filename for the stored file.
+   * @param bool $expected_as_filename
+   *   Whether the expected filename should be the filename property too.
    *
    * @return array
    *   The expected normalized data array.
    */
-  protected function getExpectedNormalizedEntity($fid = 1, $expected_filename = 'example.txt') {
+  protected function getExpectedNormalizedEntity($fid = 1, $expected_filename = 'example.txt', $expected_as_filename = FALSE) {
     $author = User::load(static::$auth ? $this->account->id() : 0);
     $file = File::load($fid);
 
@@ -364,7 +390,7 @@ protected function getExpectedNormalizedEntity($fid = 1, $expected_filename = 'e
       ],
       'filename' => [
         [
-          'value' => 'example.txt',
+          'value' => $expected_as_filename ? $expected_filename : 'example.txt',
         ],
       ],
       'uri' => [