Fix issues of no file extension for pad export

www/pad/export.js

@@ -5,7 +5,7 @@ define([
     '/bower_components/nthen/index.js',
 ], function ($, Util, Hyperjson, nThen) {
     var module = {
-        type: 'html'
+        ext: '.html'
     };
 
     var exportMediaTags = function (inner, cb) {

www/pad/inner.js

@@ -786,7 +786,7 @@ define([
             });
         }, true);
 
-        framework.setFileExporter(Exporter.type, function (cb) {
+        framework.setFileExporter(Exporter.ext, function (cb) {
             Exporter.main(inner, cb);
         }, true);