fix missing binding this

Author: peecky

lib/uploadhandler.js

@@ -51,10 +51,10 @@ module.exports = function (options) {
                     else cb(err);
                 }, this));
             }, this),
-            function(err) {
+            _.bind(function(err) {
                 if (err) console.log(err);
                 this.callback({files: files});
-            });
+            }, this));
         }, this));
     };