Builds form parameters.

        :param post_params: Normal form parameters.
        :param files: File parameters.
        :return: Form parameters with files.

def prepare_post_parameters(self, post_params=None, files=None):
        """Builds form parameters.

        :param post_params: Normal form parameters.
        :param files: File parameters.
        :return: Form parameters with files.
        """
        params = []

        if post_params:
            params = post_params

        if files:
            for k, v in six.iteritems(files):
                if not v:
                    continue
                file_names = v if type(v) is list else [v]
                for n in file_names:
                    with open(n, 'rb') as f:
                        filename = os.path.basename(f.name)
                        filedata = f.read()
                        mimetype = (mimetypes.guess_type(filename)[0] or
                                    'application/octet-stream')
                        params.append(
                            tuple([k, tuple([filename, filedata, mimetype])]))

        return params