mongo_paperclip.rb

module Paperclip
  class << self
    def logger #:nodoc:
      MongoMapper.logger
    end
  end
    
  module ClassMethods
    def has_attached_file name, options = {}
      include InstanceMethods

      write_inheritable_attribute(:attachment_definitions, {}) if attachment_definitions.nil?
      attachment_definitions[name] = {:validations => []}.merge(options)

      after_save :save_attached_files
      before_destroy :destroy_attached_files

      define_callbacks :before_post_process, :after_post_process
      define_callbacks :"before_#{name}_post_process", :"after_#{name}_post_process"
     
      define_method name do |*args|
        a = attachment_for(name)
        (args.length > 0) ? a.to_s(args.first) : a
      end

      define_method "#{name}=" do |file|
        attachment_for(name).assign(file)
      end

      define_method "#{name}?" do
        attachment_for(name).file?
      end
      
      validates_each name, :logic => lambda {|record, attr, value|
        attachment = record.attachment_for(name)
        attachment.send(:flush_errors) unless attachment.valid?
      }
    end
  end

  module Interpolations
    # Handle string ids (mongo)
    def id_partition attachment, style
      if (id = attachment.instance.id).is_a?(Integer)
        ("%09d" % id).scan(/\d{3}/).join("/")
      else
        id.scan(/.{3}/).first(3).join("/")
      end
    end
  end
end

Read more about this at: http://blog.anlek.com/2010/06/getting-paperclip-to-work-with-mongomapper/