The CDM_SETDEFEXT message sets the default filename extension for an Explorer-style Open or Save As common dialog box. The dialog box must have been created with the OFN_EXPLORER flag; otherwise, the message fails.
CDM_SETDEFEXT wParam = 0; lParam = (LPARAM) (LPSTR) pszext; // corresponding macro void CommDlg_OpenSave_SetDefExt(hdlg, pszext)
No return value.