diff --git a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
index db97169ec4..7f840fb988 100644
--- a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
+++ b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
@@ -45,6 +45,13 @@ class FileUploadResource extends ResourceBase {
   }
 
   /**
+   * The regex used to extract the filename from the content disposition header.
+   *
+   * @var string
+   */
+  const FILENAME_REGEX = '@\bfilename=\"(?<filename>.+)\"@';
+
+  /**
    * @var \Drupal\Core\File\FileSystem
    */
   protected $fileSystem;
@@ -139,7 +146,16 @@ public static function create(ContainerInterface $container, array $configuratio
    * Creates a file from endpoint.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
-   * @param \Drupal\file\FileInterface $file
+   *   The current request.
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle
+   *   The bundle.
+   * @param string $field_name
+   *   The field name.
+   *
+   * @return \Drupal\rest\ModifiedResourceResponse
+   *   A 201 response, on success.
    */
   public function post(Request $request, $entity_type_id, $bundle, $field_name) {
     $filename = $this->validateAndParseContentDispositionHeader($request);
@@ -222,16 +238,18 @@ protected function streamUploadData($destination_uri) {
   /**
    * Validates and extracts the filename from the Content-Disposition header.
    *
-   * @param string $content_disposition
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request object.
    *
    * @return string
+   *   The filename extracted from the header.
    *
    * @throws \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
    */
   protected function validateAndParseContentDispositionHeader(Request $request) {
     // Firstly, check the header exists.
     if (!$request->headers->has('content-disposition')) {
-      throw new BadRequestHttpException('"Content-Disposition" header is required. A file name in the format "filename=FILENAME" should be provided');
+      throw new BadRequestHttpException('"Content-Disposition" header is required. A file name in the format "filename=FILENAME" must be provided');
     }
 
     $content_disposition = $request->headers->get('content-disposition');
@@ -239,8 +257,8 @@ protected function validateAndParseContentDispositionHeader(Request $request) {
     // Parse the header value. This regex does not allow an empty filename.
     // i.e. 'filename=""'. This also matches on a word boundary so other keys
     // like 'not_a_filename' don't work.
-    if (!preg_match('@\bfilename=\"(?<filename>.+)\"@', $content_disposition, $matches)) {
-      throw new BadRequestHttpException('No filename found in "Content-Disposition" header. A file name in the format "filename=FILENAME" should be provided');
+    if (!preg_match(static::FILENAME_REGEX, $content_disposition, $matches)) {
+      throw new BadRequestHttpException('No filename found in "Content-Disposition" header. A file name in the format "filename=FILENAME" must be provided');
     }
 
     // Don't validate the actual filename here, that will be done by the upload
@@ -314,7 +332,7 @@ protected function validate(FileInterface $file, FieldDefinitionInterface $field
    *   The array of field settings.
    *
    * @return string
-   *   An unsanitized file directory URI with tokens replaced. The result of
+   *   An un-sanitized file directory URI with tokens replaced. The result of
    *   the token replacement is then converted to plain text and returned.
    */
   protected function getUploadLocation(array $settings) {
@@ -388,6 +406,7 @@ protected function getBaseRouteRequirements($method) {
     // incoming requests can only use the 'application/octet-stream'
     // Content-Type header.
     $requirements['_content_type_format'] = 'bin';
+    // Allow all serializer formats to be returned as a response format.
     $requirements['_format'] = implode('|', $this->serializerFormats);
 
     return $requirements;
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 94ddb67ab1..d1e65730a7 100644
--- a/core/modules/hal/tests/src/Functional/EntityResource/File/FileUploadHalJsonTestBase.php
+++ b/core/modules/hal/tests/src/Functional/EntityResource/File/FileUploadHalJsonTestBase.php
@@ -31,11 +31,10 @@
   protected function getExpectedNormalizedEntity() {
     $normalization = parent::getExpectedNormalizedEntity();
 
-    // @todo Look into why apply applyHalFieldNormalization() doesn't work here.
-    //   - It does not remove the uid property
-    //   - It add an 'en' langcode to the created property only (suggesting it
-    //     thinks just that field is translatable).
-    //   - The file URL needs to be created anyway AFAICT.
+    // Cannot use applyHalFieldNormalization() as it uses the $entity property
+    // from the test class, which in the case of file upload tests, is the
+    // parent entity test entity for the file that's created.
+
     // Hal uses the full file URL for the URI field.
     $normalization['uri'][0]['value'] = file_create_url($normalization['uri'][0]['value']);
     // Hal adds reference fields in links and embedded.
@@ -44,7 +43,7 @@ protected function getExpectedNormalizedEntity() {
     return $normalization + [
         '_links' => [
           'self' => [
-            'href' => file_create_url('public://foobar/example.txt'),
+            'href' => $normalization['uri'][0]['value'],
           ],
           'type' => [
             'href' => $this->baseUrl . '/rest/type/file/file',
diff --git a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
index c3d653bbdb..b6cb7c04c7 100644
--- a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
@@ -130,16 +130,18 @@ public function testPostFileUpload() {
     $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => '']);
     $this->assertSame(400, $response->getStatusCode());
 
-    // An empty filename in the Content-Disposition header should return a 400.
+    // 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());
 
-    // An empty filename in the Content-Disposition header should return a 400.
+    // 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());
 
-    // Another key-value pair in the Content-Disposition header should return a
-    // 400.
+    // 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());
 
@@ -253,8 +255,6 @@ protected function getExpectedNormalizedEntity() {
    * @param array $headers
    *   Additional headers to send with the request. Defaults will be added for
    *   Content-Type and Content-Disposition.
-   * @param string $content_type
-   *   The content type header to send in the file request.
    *
    * @return \Psr\Http\Message\ResponseInterface
    */