Fixed #12412 -- Clarified the documentation around file handling by the test...

        (The name ``attachment`` here is not relevant; use whatever name your

        file-processing code expects.)

        Note that you should manually close the file after it has been provided

        to ``post()``.

        Note that if you wish to use the same file handle for multiple

        ``post()`` calls then you will need to manually reset the file

        pointer between posts. The easiest way to do this is to

        manually close the file after it has been provided to

        ``post()``, as demonstrated above.

        You should also ensure that the file is opened in a way that

        allows the data to be read. If your file contains binary data

        such as an image, this means you will need to open the file in

        ``rb`` (read binary) mode.

        The ``extra`` argument acts the same as for :meth:`Client.get`.