private_assets_controller.rb

class PrivateAssetsController
  before_filter :authentificate_user!
  
  def show
    # проверка на существование файла
    if file
      send_file(file, ....)
    else
      render nothing: true, status: :not_found
    end
  end
end
    

routes.rb

....

get '/uploads/:file', to: 'private_assets#show'

...