option_a.rb

class Page < ActiveRecord::Base
  has_attached_file :image
end

class DelayedAttachment
  attr_reader :attachment

  def initialize attachment
    @attachment = attachment
  end

  def delay
    attachment.halt_processing!
    DelayedJob.enqueue SomeJob.new(attachment.instance.class, attachment.instance.id)
  end
end

class SomeJob
  def initialize klass, id
    @klass, @id = klass, id
  end

  def perform
    @klass.find(@id).reprocess!
  end
end

class PagesController < ApplicationController
  def create
    @page = Page.find params[:id]

    if @page.image_changed?
      DelayedAttachment.new(@page.image).delay
    end

    @page.save

    redirect_to page_path(@page)
  end
end