diff --git a/core/modules/hal/tests/src/Functional/EntityResource/File/FileUploadHalJsonTestBase.php b/core/modules/hal/tests/src/Functional/EntityResource/File/FileUploadHalJsonTestBase.php
index 1a02530807..735e7a5ece 100644
--- a/core/modules/hal/tests/src/Functional/EntityResource/File/FileUploadHalJsonTestBase.php
+++ b/core/modules/hal/tests/src/Functional/EntityResource/File/FileUploadHalJsonTestBase.php
@@ -28,8 +28,8 @@
   /**
    * {@inheritdoc}
    */
-  protected function getExpectedNormalizedEntity($fid = 1, $actual_filename = 'example.txt') {
-    $normalization = parent::getExpectedNormalizedEntity($fid, $actual_filename);
+  protected function getExpectedNormalizedEntity($fid = 1, $expected_filename = 'example.txt') {
+    $normalization = parent::getExpectedNormalizedEntity($fid, $expected_filename);
 
     // Cannot use applyHalFieldNormalization() as it uses the $entity property
     // from the test class, which in the case of file upload tests, is the
diff --git a/core/modules/rest/src/RequestHandler.php b/core/modules/rest/src/RequestHandler.php
index 2ee1be7dda..3de5839d3a 100644
--- a/core/modules/rest/src/RequestHandler.php
+++ b/core/modules/rest/src/RequestHandler.php
@@ -226,10 +226,8 @@ protected function denormalizeRequestData(Request $request, ResourceInterface $r
   /**
    * Prepares the REST response.
    *
-   * @TODO is there are requirement that REST resources MUST return a response
-   *   object? If so, we can type hint that here.
-   *
-   * @param $response
+   * @param \Drupal\rest\ResourceResponseInterface $response
+   *   The REST resource response.
    * @param \Drupal\rest\RestResourceConfigInterface $resource_config
    *   The REST resource config entity.
    *
diff --git a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
index aa9d0543c2..9b3d683e09 100644
--- a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
@@ -139,26 +139,26 @@ public function testPostFileUpload() {
 
     // The wrong content type header should return a 415 code.
     $response = $this->fileRequest($uri, $this->testFileData, ['Content-Type' => static::$mimeType]);
-    $this->assertSame(415, $response->getStatusCode());
+    $this->assertResourceErrorResponse(415, sprintf('No route found that matches "Content-Type: %s"', static::$mimeType), $response);
 
     // An empty Content-Disposition header should return a 400.
     $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => '']);
-    $this->assertSame(400, $response->getStatusCode());
+    $this->assertResourceErrorResponse(400, '"Content-Disposition" header is required. A file name in the format "filename=FILENAME" must be provided', $response);
 
     // An empty filename with a context in the Content-Disposition header should
     // return a 400.
     $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => 'file; filename=""']);
-    $this->assertSame(400, $response->getStatusCode());
+    $this->assertResourceErrorResponse(400, 'No filename found in "Content-Disposition" header. A file name in the format "filename=FILENAME" must be provided', $response);
 
     // An empty filename without a context in the Content-Disposition header
     // should return a 400.
     $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => 'filename=""']);
-    $this->assertSame(400, $response->getStatusCode());
+    $this->assertResourceErrorResponse(400, 'No filename found in "Content-Disposition" header. A file name in the format "filename=FILENAME" must be provided', $response);
 
     // An invalid key-value pair in the Content-Disposition header should return
     // a 400.
     $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => 'not_a_filename="example.txt"']);
-    $this->assertSame(400, $response->getStatusCode());
+    $this->assertResourceErrorResponse(400, 'No filename found in "Content-Disposition" header. A file name in the format "filename=FILENAME" must be provided', $response);
 
     // This request will have the default 'application/octet-stream' content
     // type header.
@@ -175,7 +175,7 @@ public function testPostFileUpload() {
   }
 
   /**
-   * Tests using the file upload POST route with a duplicate file.
+   * Tests using the file upload POST route with a duplicate file name.
    *
    * A new file should be created with a suffixed name.
    */
@@ -200,7 +200,7 @@ public function testPostFileUploadDuplicateFile() {
 
     $this->assertSame(201, $response->getStatusCode());
 
-    // Loading expected normalized data for file 2 and duplicate file.
+    // Loading expected normalized data for file 2, the duplicate file.
     $expected = $this->getExpectedNormalizedEntity(2, 'example_0.txt');
 
     $this->assertResponseData($expected, $response);
@@ -251,7 +251,7 @@ public function testFileUploadInvalidFileType() {
     // Test with a JSON file.
     $response = $this->fileRequest($uri, '{"test":123}', ['Content-Disposition' => 'filename="example.json"']);
 
-    $this->assertSame(422, $response->getStatusCode());
+    $this->assertResourceErrorResponse(422, "Unprocessable Entity: file validation failed.\nOnly files with the following extensions are allowed: <em class=\"placeholder\">txt</em>.", $response);
 
     // Make sure that no file was saved.
     $this->assertEmpty(File::load(1));
@@ -278,7 +278,7 @@ public function testFileUploadLargerFileSize() {
     // Generate a string larger than the 50 byte limit set.
     $response = $this->fileRequest($uri, $this->randomString(100));
 
-    $this->assertSame(422, $response->getStatusCode());
+    $this->assertResourceErrorResponse(422, "Unprocessable Entity: file validation failed.\nThe file is <em class=\"placeholder\">100 bytes</em> exceeding the maximum file size of <em class=\"placeholder\">50 bytes</em>.", $response);
 
     // Make sure that no file was saved.
     $this->assertEmpty(File::load(1));
@@ -304,13 +304,13 @@ protected function getExpectedUnauthorizedAccessMessage($method) {
    *
    * @param int $fid
    *   The file ID to load and create normalized data for.
-   * @param string $actual_filename
-   *   The actual filename for the stored file.
+   * @param string $expected_filename
+   *   The expected filename for the stored file.
    *
    * @return array
    *   The expected normalized data array.
    */
-  protected function getExpectedNormalizedEntity($fid = 1, $actual_filename = 'example.txt') {
+  protected function getExpectedNormalizedEntity($fid = 1, $expected_filename = 'example.txt') {
     $author = User::load(static::$auth ? $this->account->id() : 0);
     $file = File::load($fid);
 
@@ -345,7 +345,7 @@ protected function getExpectedNormalizedEntity($fid = 1, $actual_filename = 'exa
       ],
       'uri' => [
         [
-          'value' => 'public://foobar/' . $actual_filename,
+          'value' => 'public://foobar/' . $expected_filename,
         ],
       ],
       'filemime' => [