module ActionMailer::Previews::ClassMethods

def register_preview_interceptor(interceptor)

string is passed in it will be constantized.
Either a class or a string can be passed in as the Interceptor. If a
Register an Interceptor which will be called before mail is previewed. 
def register_preview_interceptor(interceptor)
  preview_interceptor = case interceptor
    when String, Symbol
      interceptor.to_s.camelize.constantize
    else
      interceptor
    end
  unless preview_interceptors.include?(preview_interceptor)
    preview_interceptors << preview_interceptor
  end
end

def register_preview_interceptors(*interceptors)

Register one or more Interceptors which will be called before mail is previewed. 
def register_preview_interceptors(*interceptors)
  interceptors.flatten.compact.each { |interceptor| register_preview_interceptor(interceptor) }
end