Currently you have to use a separate reports processor for Foreman, since Foreman apparently expects the YAML report data in application/x-www-form-urlencoded (the report processor template at https://raw.github.com/theforeman/puppet-foreman/master/templates/foreman-report.rb.erb calls set_form_data). Puppet reference docs also seem to suggest that Puppet sends the data in urlencoded format, but that is not the case: see https://projects.puppetlabs.com/issues/14723

If Foreman accepted YAML-formatted data as the request body, it would work with the Puppet http report processor and separate processors would no longer be needed.