Issue #3618271 by luigisa: Register /flysystem/files/private for overridden private://

Luisrequested to merge
issue/flysystem-3618271:3618271-overriding-private-with into 3.0.x

getExternalUrl() emits Flysystem download URLs for private visibility, but core's FileDownloadController cannot stream remote S3 objects.

Co-authored-by: Cursor cursoragent@cursor.com

Closes #3618271

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