Remove tmp prefix from filename.

def remove_tmp_prefix_from_filename(filename):
    """
    Remove tmp prefix from filename.
    """
    if not filename.startswith(dju_settings.DJU_IMG_UPLOAD_TMP_PREFIX):
        raise RuntimeError(ERROR_MESSAGES['filename_hasnt_tmp_prefix'] % {'filename': filename})
    return filename[len(dju_settings.DJU_IMG_UPLOAD_TMP_PREFIX):]