Currently we have two helpers available for plugin to extend allowed helpers in template rendering. Unfortunately an extra effort is required for template previewing, the concern has to be included in TemplatesController and all its children. The plugin API should do this, ideally something like

extend_template_helpers MyPlugin::Helpers

which would do following

::Foreman::Renderer.send :include, MyPlugin::Helpers
(TemplatesController.descendants + [TemplatesController]).each do |klass|
  klass.send(:include, MyPlugin::Helpers)
end

it could also call allowed_template_helpers with all public methods found in MyPlugin::Helpers.