paperclip自定义上传文件名

http://www.geilibuy.com/cod_area
http://blog.wyeworks.com/2009/7/13/paperclip-file-rename
https://github.com/thoughtbot/paperclip/wiki/Interpolations
http://yuan.javaeye.com/blog/604174

  has_attached_file :i2,
    :processors => [:jcropper],
    :styles=>{:mini=>"40x40#",:thumb=>"55x55#",:small=>"100x100#",:medium=>"160x160#",:big=>"400x400#",:process=>"800x800>"},
    :url => "/system/:class/:id_partition/:style/:filename",
    :path => ":rails_root/public/system/:class/:id_partition/:style/:filename"