Issue #3239761 by alexpott, andypost: Fix MimeTypeTest to prevent deprecations...

Issue #3239761 by alexpott, andypost: Fix MimeTypeTest to prevent deprecations in PHP 8.1 and fix \Drupal\Core\StreamWrapper\PrivateStream::basePath() documentation

core/lib/Drupal/Core/StreamWrapper/PrivateStream.php

@@ -56,8 +56,9 @@ public function getExternalUrl() {
    * so you should alter that form or substitute a different form if you change
    * the class providing the stream_wrapper.private service.
    *
-   * @return string
-   *   The base path for private://.
+   * @return string|null
+   *   The base path for private://. NULL means the private directory is not
+   *   set.
    */
   public static function basePath() {
     return Settings::get('file_private_path');

core/tests/Drupal/KernelTests/Core/File/FileTestBase.php

@@ -65,14 +65,17 @@ public function register(ContainerBuilder $container) {
    */
   protected function setUpFilesystem() {
     $public_file_directory = $this->siteDirectory . '/files';
+    $private_file_directory = $this->siteDirectory . '/private';
 
     require_once 'core/includes/file.inc';
 
     mkdir($this->siteDirectory, 0775);
     mkdir($this->siteDirectory . '/files', 0775);
+    mkdir($this->siteDirectory . '/private', 0775);
     mkdir($this->siteDirectory . '/files/config/sync', 0775, TRUE);
 
     $this->setSetting('file_public_path', $public_file_directory);
+    $this->setSetting('file_private_path', $private_file_directory);
     $this->setSetting('config_sync_directory', $this->siteDirectory . '/files/config/sync');
   }