返回的byte转为带单位的大小

 private static String fileSizeConver(long fileS) {
    DecimalFormat df = new DecimalFormat("#.00");
    String fileSizeString = "";
    String wrongSize = "0B";
    if (fileS == 0) {
      return wrongSize;
    }
    if (fileS < 1024) {
      fileSizeString = df.format((double) fileS) + "B";
    } else if (fileS < 1048576) {
      fileSizeString = df.format((double) fileS / 1024) + "KB";
    } else if (fileS < 1073741824) {
      fileSizeString = df.format((double) fileS / 1048576) + "MB";
    } else {
      fileSizeString = df.format((double) fileS / 1073741824) + "GB";
    }
    return fileSizeString;
  }