I had this document on hand, and to the best of my knowledge followed it, when upgrading.
What I see is this:
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new posts are handled correctly.
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in my database backup prior to upgrading, posts with attached files do contain the filenames.
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after the upgrade, some of these posts records do not have anything in either the filenames or fileids.
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there is no fileinfo record for these posts either.
According to the code, fileinfo records are created lazily on access instead of part of the migration. I’m not sure what the issue is - some of my file references are updated, and some are not. I don’t know why the filenames field would be set to ‘[]’ for some posts when no fileinfo record had been (lazily?) created for it.
My issue aside, I question whether it’s wise to use this lazy migration technique instead of biting the bullet and decisively migrating them at the time of system upgrade.