initialize MultipartEncoder with uploading fields.

def multipart_encoder(**kwargs):
    """ initialize MultipartEncoder with uploading fields.
    """
    def get_filetype(file_path):
        file_type = filetype.guess(file_path)
        if file_type:
            return file_type.mime
        else:
            return "text/html"

    fields_dict = {}
    for key, value in kwargs.items():

        if os.path.isabs(value):
            _file_path = value
            is_file = True
        else:
            global PWD
            _file_path = os.path.join(PWD, value)
            is_file = os.path.isfile(_file_path)

        if is_file:
            filename = os.path.basename(_file_path)
            with open(_file_path, 'rb') as f:
                mime_type = get_filetype(_file_path)
                fields_dict[key] = (filename, f.read(), mime_type)
        else:
            fields_dict[key] = value

    return MultipartEncoder(fields=fields_dict)