Resolve #3497796 "File download"

Closes #3497796

core/lib/Drupal/Core/File/file.api.php

@@ -23,7 +23,7 @@
  * @param string $uri
  *   The URI of the file.
  *
- * @return string[]|int
+ * @return string[]|int|null
  *   If the user does not have permission to access the file, return -1. If the
  *   user has permission, return an array with the appropriate headers. If the
  *   file is not controlled by the current module, the return value should be
@@ -31,7 +31,7 @@
  *
  * @see \Drupal\system\FileDownloadController::download()
  */
-function hook_file_download($uri) {
+function hook_file_download($uri): array|int|null {
   // Check to see if this is a config download.
   $scheme = StreamWrapperManager::getScheme($uri);
   $target = StreamWrapperManager::getTarget($uri);
@@ -40,6 +40,7 @@ function hook_file_download($uri) {
       'Content-disposition' => 'attachment; filename="config.tar.gz"',
     ];
   }
+  return NULL;
 }
 
 /**