Handle encoded filenames per RFC6266. See also:
        http://tools.ietf.org/html/rfc2231#section-4

def get_encoded_filename(self, filename_parm):
        """
        Handle encoded filenames per RFC6266. See also:
        http://tools.ietf.org/html/rfc2231#section-4
        """
        encoded_filename = force_text(filename_parm['filename*'])
        try:
            charset, lang, filename = encoded_filename.split('\'', 2)
            filename = urlparse.unquote(filename)
        except (ValueError, LookupError):
            filename = force_text(filename_parm['filename'])
        return filename