If there’s not an obvious solution to get it to upload successfully, either of options 1 or 3 seem like good ways to go. 2 I would probably leave to Discourse, specifically the job to clean up orphaned uploads, the interval for which is controlled by the clean orphan uploads grace period hours setting.
It might be a good idea to grab the file before it’s deleted though. If it doesn’t contain any sensitive data, it could be useful for testing why it can’t be uploaded during the migration.