Modify template attributes in preprocess

/**
* Implements hook_preprocess_HOOK().
*/
function MYMODULE_preprocess_file_video(&$variables) {
 if (isset($variables['files'][0]['source_attributes'])) {
   /** @var \Drupal\Core\Template\Attribute $attributes */
   $attributes = $variables['files'][0]['source_attributes'];
   $src = $attributes->offsetGet('src')?->value();
   $src .= '#t=0.001';
   $attributes->setAttribute('src', $src);
 }
}