try {
if (! ftpClient.changeWorkingDirectory(ftpPath))
throw new RuntimeException("Cann't ftpClient.changeWorkingDirectory('"+ ftpPath +"').");
FTPFileFilter filter = null;
if (fileExt != null) {
final String suffix = "." + fileExt;
filter = new FTPFileFilter(){
@Override
public boolean accept(FTPFile file) {
if (! file.isFile()) return false;
return file.getName().endsWith(suffix);
}