Fix adding files to exercises (as admin)

Prevent hidden selection field from being removed after cloning the dummy. Also ensure the caret looks nice and add the file extension if possible

app/assets/javascripts/exercises.js.erb

@@ -66,7 +66,6 @@ $(document).on('turbolinks:load', function() {
     $('#files').append(html);
     $('#files li:last select[name*="file_type_id"]').val(getSelectedExecutionEnvironment().file_type_id);
     $('#files li:last select').chosen(window.CodeOcean.CHOSEN_OPTIONS);
-    $('#files li:last select').remove();
     $('#files li:last>div:last').removeClass('in').addClass('show')
     $('body, html').scrollTo('#add-file');