post_process.rb

    def post_process(*style_args) #:nodoc:
      return if @queued_for_write[:original].nil?

      catch :validation_failed do
        instance.run_paperclip_callbacks(:post_process) do
          instance.run_paperclip_callbacks(:"#{name}_post_process") do
            throw :validation_failed if @options[:check_validity_before_processing] && instance.errors.any?
            post_process_styles(*style_args)
          end
        end
      end
    end