Filter for file extension.

 // Create OpenFileDialog  on button click Event!
    Microsoft.Win32.OpenFileDialog dlg = new Microsoft.Win32.OpenFileDialog();
 

 
    // Set filter for file extension.
   
    dlg.Filter = "EXCEL Files (*.xls)|*.xlsx"; 

   dlg.Filter = "Text Files | *.txt"; 

dlg .Filter = "png files (*.png)|*.png|

                          jpg files (*.jpg)|*.jpg|

                          jpeg files (*.jpeg)|*.jpeg|

                          gif files (*.gif)|*.gif|

                          Image Files(*.png;*.jpg;*.jpeg;*.gif)|*.png;*.jpg;*.jpeg;*.gif";