...use the code as per requirements.
CFileDialog mydialog( FALSE,".txt", NULL,
OFN_OVERWRITEPROMPT |OFN_EXTENSIONDIFFERENT |OFN_LONGNAMES|OFN_NOCHANGEDIR,
"Text File (*.txt)|*.txt|Estimate Files (*.det)|*.det|Report Files (*.rep)|*.rep|All (*.*)|*.*||",NULL);
Bye...