how do I add a progress bar. I use the code provided by eric bidelman here => [Polymer Core-Ajax File upload](https://stackoverflow.com/questions/25099938/polymer-core-ajax-file-upload)

```
downloadinit: function( event, detail, sender ){
            this.files = sender.files;
            for ( var i = 0; i < this.files.length; i++ ) {
                var file = this.files[i]
                this.uploadsize += file.size;
                this.fileList[this.uploadPop] = file.name;
                this.uploadPop += 1;
            }

            event.stopPropagation(); // Stop stuff happening
            event.preventDefault(); // Totally stop stuff happening

            for( var i = 0, f; f = this.files[i]; ++i) {
                this.data.append( sender.name, f, f.name );
            }
            this.queryUrl2 = "projects/submit.php?pjid="+this.projDetails.projectid;
            this.submitBody = this.data;
            this.$.ajaxAction2.contentType = null;
        },
        triggerDownload: function(){
            this.$.ajaxAction2.go();
            data = new FormData;
            this.files = null;
            this.fileList = [];
            this.submitBody = null;
            this.uploadsize = 0;
            this.uploadPop = 0;
        },

```