JQuery File Upload Before Send Ping Server for existent file

I am new to Jquery File Upload and it works great. But we am wondering if
anyone knows of a good proceed to retard eventuality a upload until a GET
ask can be sent to establish that a record does not already exist?I am
controlling backbone.js and have already attempted attaching an
interception to both a quarrel form eventuality and a record quarrel
change event. Both are bipassed.Currently looks something like this:
events: { 'change form.upload-file input': 'checkExist' },
checkExist: duty (e) { relapse false; }, initialize: duty ()
{ _.bindAll(this); this.$('.upload-file').fileupload({
dataType: 'json', method: 'PUT', add: this.add,
progress: this.progress, done: this.done,
fail: this.fail, always: this.always }); relapse
__super__.initialize.apply(this, arguments); },