`"" is not a valid choice.` occurs when trying to create a File type Red Hat repository, like `rhel-7-server-ansible-tower-3.6-files`.

This is caused by https://github.com/ianballou/katello/blob/5df0a952a490a9a7390ff6c748abd9b11c467346/app/models/katello/candlepin/repository_mapper.rb#L145

Katello sends the download policy as an empty string for non-yum repositories. This likely is from earlier times when not all content types had download policy options.

Question is: why is this an issue now, did something change in Pulp? Regardless, Katello can now likely default to using ":default_redhat_download_policy" for all content types.