Fix issues of no file extension for pad export

www/kanban/export.js

@@ -2,7 +2,9 @@
 // Pads from the code app will be exported using this format instead of plain text.
 define([
 ], function () {
-    var module = {};
+    var module = {
+        ext: '.json'
+    };
 
     module.main = function (userDoc, cb) {
         var content = userDoc.content;

www/kanban/inner.js

@@ -367,7 +367,7 @@ define([
             });
         }
 
-        framework.setFileExporter('json', function () {
+        framework.setFileExporter('.json', function () {
             return new Blob([JSON.stringify(kanban.getBoardsJSON(), 0, 2)], {
                 type: 'application/json',
             });