core/modules/file/src/FileFieldUploader.php                       | 6 ++++--
 core/modules/file/src/Plugin/rest/resource/FileUploadResource.php | 2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/core/modules/file/src/FileFieldUploader.php b/core/modules/file/src/FileFieldUploader.php
index cf45884166..4b3f011df1 100644
--- a/core/modules/file/src/FileFieldUploader.php
+++ b/core/modules/file/src/FileFieldUploader.php
@@ -35,7 +35,9 @@
  *   - Permission to upload a file can be determined by a user's field- and
  *     entity-level access.
  *
- * @internal
+ * @internal This is not a public API and subject to change until the
+ *   underlying File API is cleaned up in
+ *   https://www.drupal.org/project/drupal/issues/3021652.
  */
 class FileFieldUploader {
 
@@ -222,7 +224,7 @@ public function handleFileUploadForField(FieldDefinitionInterface $field_definit
    * @throws \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
    *   Thrown when the 'Content-Disposition' request header is invalid.
    */
-  public static function validateAndParseContentDispositionHeader(Request $request) {
+  public function validateAndParseContentDispositionHeader(Request $request) {
     // First, check the header exists.
     if (!$request->headers->has('content-disposition')) {
       throw new BadRequestHttpException('"Content-Disposition" header is required. A file name in the format "filename=FILENAME" must be provided.');
diff --git a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
index de49dfc120..cd6056f741 100644
--- a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
+++ b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
@@ -156,7 +156,7 @@ public function permissions() {
   public function post(Request $request, $entity_type_id, $bundle, $field_name) {
     $field_definition = $this->validateAndLoadFieldDefinition($entity_type_id, $bundle, $field_name);
 
-    $filename = FileFieldUploader::validateAndParseContentDispositionHeader($request);
+    $filename = $this->fileFieldUploader->validateAndParseContentDispositionHeader($request);
     $stream = FileFieldUploader::getUploadStream();
     $file = $this->fileFieldUploader->handleFileUploadForField($field_definition, $filename, $stream, $this->currentUser);
     fclose($stream);