spec_helper.rb

require 'rubygems'
require 'paperclip/matchers'

def cancel_paperclip_post_processing(cls)
  cls.class_eval do
    before_post_process :cancel_post_processing

    def cancel_post_processing
      false
    end
  end
end

# ... other stuff here

yourclass_spec.rb

require 'spec_helper'

cancel_paperclip_post_processing YourClass

describe YourClass do
  
  describe "validations" do
    
    it "should require a valid contract file (if supplied)" do
      YourClass.should have_attached_file(:contract)
      YourClass.should validate_attachment_content_type(:contract).allowing('application/pdf').rejecting('application/msword', 'application/rtf')
      YourClass.should validate_attachment_size(:contract).less_than(5.megabytes)
    end
end