code: protected function validateExtension($file)
      {
          $extension = mb_strtolower($file->extension, 'UTF-8');
  
          if ($this->checkExtensionByMimeType) {
              $mimeType = FileHelper::getMimeType($file->tempName, null, false);
              if ($mimeType === null) {
                  return false;
              }
  
              $extensionsByMimeType = FileHelper::getExtensionsByMimeType($mimeType);
  
              if (!in_array($extension, $extensionsByMimeType, true)) {
                  return false;
              }
          }
  
          if (!in_array($extension, $this->extensions, true)) {
              return false;
          }
  
          return true;
      }
docs: Checks if given uploaded file have correct type (extension) according current validator settings.
  @param UploadedFile $file
  @return bool