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In Drupal 7, it was possible under
admin/config/media/file-system to set the "Public file system path". This setting is no longer available from the UI in Drupal 8.
In Drupal 7, it was also possible to change this by setting
settings.php. In Drupal 8, it is now called
If you changed this setting in Drupal 7 then before upgrading you need to add
$settings['file_public_path'] = 'path/to/the/files/directory';