[backup-import] Wrong file ownership for sites, imported from backups, causing broken file attachments

Steps to reproduce

1) Backup a site that has a content type with file attachments.
2) Import the backup following the official manual.

What should happen

According to documentation, everything under sites/example.com/files/ should belong to www-data:www-data. Ditto for sites/example.com/private/.

What actually happens

The content of sites/example.com/files/ and sites/example.com/private/ belongs to aegir:www-data, while permissions remain correct: drwxr-sr-x for subdirectories.

As a result, file uploads fail with:

The upload directory public://attachments for the file field field_attachments could not be created or is not accessible. A newly uploaded file could not be saved in this directory as a consequence, and the upload was canceled.