My controller accesses the tempfile attribute of an uploaded file and passes it to another mocked component. My test code has
  @file = mock(Object)         
        I made a pull request to fix this issue, please +1 if you like it: https://github.com/brynary/rack-test/pull/67
I went around this way
upload_file = fixture_file_upload('files/stats_upload.csv', 'text/csv')
upload_file.stubs(:tempfile).returns(upload_file)
                                                                        Finally found this, which tells that although the thing returned by fixture_file_upload has a @tempfile member, it lacks the reader method. Solved as follows
  FileUtils.touch('file.zip') # fixture_file_upload needs the file to exist
  @file = fixture_file_upload('file.zip')
  class << @file
    # The reader method is present in a real invocation,
    # but missing from the fixture object for some reason (Rails 3.1.1)
    attr_reader :tempfile
  end