Fix issues of no file extension for pad export

www/whiteboard/export.js

@@ -14,7 +14,7 @@ define([
         var canvas = new Fabric.Canvas(canvas_node);
         var content = userDoc.content;
         canvas.loadFromJSON(content, function () {
-            module.type = 'svg';
+            module.ext = '.svg';
             cb(canvas.toSVG());
         });
     };

www/whiteboard/inner.js

@@ -415,7 +415,7 @@ define([
             setEditable(!locked);
         });
 
-        framework.setFileExporter('png', function (cb) {
+        framework.setFileExporter('.png', function (cb) {
             $canvas[0].toBlob(function (blob) {
                 cb(blob);
             });