Problem in testing with separate validation of :attachment_file_name

Using: RSpec, Shoulda, Paperclip (including matchers)

Example Model

class Example < ActiveRecord::Base
  has_attached_file :attachment

  validates_attachment_presence :attachment
  validates_format_of :attachment_file_name, :with => /\.(png|jpe?g)$/
end

Example Test

require 'spec/helper'

describe Example do
  it { should validate_attachment_presence(:attachment) }

  it { should validate_format_of(:attachment_file_name).
          with('example.png') }

  it { should validate_format_of(:attachment_file_name).
          not_with('example.zip').
          with_message(/is invalid/) }
end

This causes my validates_attachment_presence(:attachment) test to fail.

If I remove the validates_format_of :attachment_file_name line from my
model, the presence test now passes, but, obviously, the format test
fails.

I noticed that basically all validates_attachment_presence does is
require attachment_file_name, but I can't seem to find any reason why it
affects my format tests.

I am not sure if this is necessarily Paperclip issue (as apposed to
perhaps a Shoulda one), but hopefully this is a good enough report to
explain the issue.