Fixed issue #54.

Author: killme2008

lib/file.js

@@ -505,12 +505,14 @@
           if(AV._isNode){
             //Use qiniu sdk to upload files to qiniu.
             var qiniu = require('qiniu');
+            var path = require('path');
             self._previousSave = self._source.then(function(base64, type) {
               //Create 16-bits uuid as qiniu key.
               var hexOctet = function() {
                 return Math.floor((1+Math.random())*0x10000).toString(16).substring(1);
               };
-              var key = hexOctet() + hexOctet() + hexOctet() + hexOctet();
+              var key = hexOctet() + hexOctet() + hexOctet() + hexOctet()
+                          + path.extname(self._name);
               var data = {
                 key: key,
                 ACL: self._acl,