This function frees disk space on your hosting account. It will scan your uploads directory (recursevely) and check every found file whether it is wp attachment or no. All files that are not attachments (no matter got parents or no) will de deleted. [Wordpress]

clean_uploads_from_nonattachments.php

//Delete orphaned files in wp-content/uploads that are not wordpress attachments
function clean_uploads_from_nonattachments(){
	$uploads_dir = wp_upload_dir(); 
	$search = $uploads_dir['basedir'];
	$replace = $uploads_dir['baseurl'];
	//You may want to take it by bites if your uploads is rather large (over 5 gb for example)
	//$uploads_dir = ( $uploads_dir['basedir'] . '/2015/' ); 
	$uploads_dir = ( $uploads_dir['basedir']); 
	$root = $uploads_dir;
	//Going through directiry recursevely
	$iter = new RecursiveIteratorIterator(
		new RecursiveDirectoryIterator($root, RecursiveDirectoryIterator::SKIP_DOTS),
		RecursiveIteratorIterator::SELF_FIRST,
		RecursiveIteratorIterator::CATCH_GET_CHILD // Ignore "Permission denied"
	);
	foreach ($iter as $fileinfo) {
		//get files only
		if ($fileinfo->isFile()) {	
			$image = $fileinfo->getPathname();
			$image_url = str_replace($search, $replace, $image);
			//Core WP function to retrieve attachment ID by URL
			$attachment_id = attachment_url_to_postid($image_url);
				//Not found - then delete file 
				if (!$attachment_id){
					unlink($image);	
				}
				else {
					//List of found attachments
					echo $attachment_id.': '.$image; 			   
				}	
		}
	}			
}