gistfile1.rb

##
# From: http://d.hatena.ne.jp/falkenhagen/20090702/1246503899
#
# Quick-n-dirty helper module method for testing the results of 
# using #send_file in your controllers
##

module SendFileHelper
  def file_body
    output = StringIO.new
    output.binmode
    
    assert_nothing_raised do
      @response.body.call(@response, output)
    end 

    output.string
  end
end

## And to use:

def test_send_file_is_good
  get :action
  assert_equals "File Body as String", file_body
end