how tp get just the extension of a file

Hi I have this code

file_name = form_file_input.getName();
file_size = (int) form_file_input.getSize();
file_type = form_file_input.getContentType();

Now form_file_input.getContentType(); gets this value application/vnd.ms-excel for an excel file
but what I need is to get .xls so that I can add this .xls to the filename when I am saving the file.

Any help will be greatly appreciated