fields = {
                'foo': 'bar',
                'fakefile': ('foofile.txt', 'contents of foofile'),
                'realfile': ('barfile.txt', open('realfile').read()),
                'typedfile': ('bazfile.bin', open('bazfile').read(),
                              'image/jpeg'),
                'nonamefile': 'contents of nonamefile field',
            }

        When uploading a file, providing a filename (the first parameter of the
        tuple) is optional but recommended to best mimic behavior of browsers.

        Note that if ``headers`` are supplied, the 'Content-Type' header will
        be overwritten because it depends on the dynamic random boundary string
        which is used to compose the body of the request. The random boundary
        string can be explicitly set with the ``multipart_boundary`` parameter.
        Nr