gistfile1.rb

class Stamp < ActiveRecord::Base
  Paperclip.interpolates :img_dir_num do |attachment, style|
    (attachment.instance.id * 0.01).to_i
  end
  Paperclip.interpolates :filename do |attachment, style|
    attachment.instance.id.to_s + ".png"
  end
  if Rails.env.production?
    S3_CREDENTIALS = {access_key_id: ENV['S3_ACCESS_KEY_ID'], secret_access_key: ENV['S3_SECRET_KEY'], bucket: "hoge-bucket"}
  end
  if Rails.env.production?
    has_attached_file :stamp,
      storage: :s3,
      s3_credentials: S3_CREDENTIALS,
      styles: {stamps_s: "100x100>", stamps:"350x350>"},
      url: ":s3_domain_url",
      path: "stamps/:style/:img_dir_num/:filename"
  else
    has_attached_file :stamp,
      url: "/:style/:img_dir_num/:filename" ,
      styles: {stamps_s: "100x100>", stamps:"350x350>"}
  end
end

これもスペーシングだけ
https://gist.github.com/hito-asa/5290947