private byte[] getFileExtNameByte(String fileName) {
		String fileExtName = null;
		int nPos = fileName.lastIndexOf('.');
		if (nPos > 0 && fileName.length() - nPos <= FDFS_FILE_EXT_NAME_MAX_LEN + 1) {
			fileExtName = fileName.substring(nPos + 1);
		}
		if (fileExtName != null && fileExtName.length() > 0) {
			return fileExtName.getBytes(charset);
		}else{
			return new byte[0];
		}
	}