Create a topic in your board ("Attachments to be deleted") and attach all orphaned attachment to that topic
Tell your users to check that topic for attachments they want to claim; if they are finished, just delete the topic.
(but 99.99% you don't need, because the result from upload or posting errors)
Edit: Or, use the script from the KB article Knowledge Base - Backing up attachments with their original filenames
and look only for "is_orphaned" to copy them to a folder