For anyone reading this thread in the future see
https://vborg.vbsupport.ru/showthread.php?p=2429574
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My idea related to this is to somehow make use of the attachments.php file provided here. Then set up a cron job to run every night using
http://s3tools.org/s3cmd-sync to transfer all newly uploaded attachments to my Amazon s3 account and another cron job to delete all image attachments older than, say, 2 weeks.
That way I will only have to store 2 weeks' worth of attachments on my server.