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<?php 

$result = db_query("
SELECT fm.*
FROM 	file_managed AS fm
		   LEFT OUTER JOIN file_usage AS fu ON ( fm.fid = fu.fid )
		   LEFT OUTER JOIN node AS n ON  fu.id::integer = n.nid 
WHERE (fu.type = 'node' OR fu.type IS NULL) AND n.nid IS NULL
ORDER BY fm.fid  DESC");

//Delete file & database entry
foreach ($result as $delta => $record) {
      file_delete($record->fid);
}



$file = \Drupal\file\Entity\File::load($fid);
$file_usage = \Drupal::service('file.usage');
$list = $file_usage->listUsage($file);