https://blog.faceutil.com/2017/05/22/android-down-file-using-ftpftp%eb%b0%a9%ec%8b%9d%ec%9c%bc%eb%a1%9c-%ed%8c%8c%ec%9d%bc-%eb%8b%a4%ec%9a%b4-%e3%82%91/