Enhance 'beforeupload' event

Hi guys, I'm using your project to upload files directly to S3. However before XHR data can be submited I need to send a callback to my back-end API in order to receive correct credentials for the file. 'beforeupload' event doesn't allow to wait until I receive response from my server. So, I propose to handle response from 'beforeupload' and if it is false then data will be submitted only after explicit call to item.submitData() is made.