gistfile1.rb

def file_upload_from_string(string, content_type = 'text/plain')
  test_upload_file = nil
  Tempfile.open('file_upload_from_string') do |tempfile|
    tempfile.write(string)
    tempfile.rewind
    test_upload_file = ActionController::TestUploadedFile.new(tempfile.path, content_type)
  end
  return test_upload_file
end

# post :create, :my_params => {:uploaded_file => file_upload_from_string('foo')}