Managed File Field and file_usage_add() - what should I attach the file to?
I attached the file to user 1 and can't see any adverse effects as yet:

file_usage_add($file, 'mantis', 'user', 1);


I also OMITTED the following line from my submit function:

$file-&gt;status = FILE_STATUS_PERMANENT;


This means that the file will be deleted after 6 hours when cron runs.  

If anyone has a better solution then I'll be happy to mark as answer.