Generating an attachment of a private image in Rails is done by calling the cl_private_download_url view helper method, for example in Rails:
<%= link_to("Download", cl_private_download_url("my_private_image", :jpg, :attachment => true)) %>
Or by using Cloudinary::Utils.private_download_url from your model or controller, for example:
Cloudinary::Utils.private_download_url self.cloudinary_id, self.format, attachment: true