you can use like this.
string fileFilter = "Excel Workbook (*.xlsx),*.xlsx," +
"Excel Macro-Enabled Workbook (*.xlsm),*.xlsm," +
"Excel 97-2003 Workbook (*.xls),*.xls," +
"Excel Template (*.xltx),*.xltx," +
"Excel Macro-Enbaled Template (*.xltm),*.xltm," +
"CSV (Comma delimited)(*.csv),*.csv,";
object sa = Wb.Application.GetSaveAsFilename(m, fileFilter.TrimEnd(new char[] { ',' }), m, m, m);
or VB code like this
fileSaveName = Application.GetSaveAsFilename( _
fileFilter:="Text Files (*.txt), *.txt")
If fileSaveName <> False Then
MsgBox "Save as " & fileSaveName
End If