You could use something like this...

CComboBox *pWndFilesOfTypeCombo = (CComboBox*)GetParent()->GetDlgItem(cmb1);

int index = pWndFilesOfTypeCombo->GetCurSel();

CString ext;
ext.SetSysString((BSTR)pWndFilesOfTypeCombo->GetItemData(index));

That will get you the index of the selected item and the extension of that selection.

Dan