The primary means of protecting the files is the CSPRN appended to the filename, but there is no reason to keep the files after the data subject has downloaded them, so deleting them provides an additional layer of protection. Previously this was done from `wp_privacy_generate_personal_data_export_file()`, but that does not guarantee that it will be run regularly, and on smaller sites that could result in export files being exposed for much longer than necessary.
`wp_privacy_delete_old_export_files()` was moved to a front end file, so that it can be called from `cron.php`.
This introduces the `wp_privacy_export_expiration` filter, which allows plugins to customize how long the exports are kept before being deleted.
`index.html` was added to the `$exclusions` parameter of `list_files()` to make sure that it isn't deleted. If it were, then poorly-configured servers would allow the directory to be traversed, exposing all of the exported files.
Props iandunn, desrosj.
See #43546.
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