You might find a reference to which file failed via Admin > Logs > Error Logs
If that doesn’t help, I think searching the database for posts with cooked that contain yourdomain/uploads/ will get you a list of those 19 posts mentioned in the migration status. Worst case, you could then manually look at those and compare any uploads with what’s in S3.
Something like:
rails c
Post.where("cooked like '%discourse.example.com/uploads/%'")