title: Save dialog doesn't add extension to file
body: <p>I have a button that opens a save dialog window with default extension filters set. When I save a file it creates it in the correct folder, but it doesn't add the extension on the end. How would I go about adding the extension to the end of the file based on the filter?
  Here is my save file button code:</p>
  
  <pre><code>    btnNewButton = new JButton("Export File");
      btnNewButton.addActionListener (new ActionListener() {
  
          public void actionPerformed(ActionEvent arg0) {
              final JFileChooser finder = new JFileChooser();
              finder.setFileFilter(new FileNameExtensionFilter("Board Files", "boa"));
              int returnVal = finder.showSaveDialog(null);
              if (returnVal == javax.swing.JFileChooser.APPROVE_OPTION) {
                  java.io.File file = finder.getSelectedFile();
                  String file_name = file.toString();
                  JOptionPane.showMessageDialog(null, file_name);
                  WriteFile data = new WriteFile(file_name);              
                  try {
                      data.writeToFile("Testing 1");
                  } catch (IOException e) {
                      System.out.println(e.getMessage());
                  }
              }
          }
      });
  </code></pre>