def _get_upload_headers(first_byte, file_size, chunk_size):
    """Prepare the string for the POST request's headers."""

    content_range = 'bytes ' + \
        str(first_byte) + \
        '-' + \
        str(first_byte + chunk_size - 1) + \
        '/' + \
        str(file_size)

    return {'Content-Range': content_range}