simultaneous fileupload media lost

Steps to Reproduce

run the fiddle: http://zkfiddle.org/sample/1hjcgam/3-simultaneous-upload-media-lost

<zk>
	<button label="upload" upload="true" onUpload='Clients.log("Uploaded: " + event.getMedia());'/>
</zk>

start several concurrent uploads quickly, choose files long enough or limit network speed in developer tools

Current Result

sometimes the uploadevent.getMedia() will return null

Uploaded: org.zkoss.zk.au.http.AuDropUploader$StreamVideo@2c3bc10b
Uploaded: org.zkoss.zk.au.http.AuDropUploader$StreamVideo@3d1a5838
Uploaded: org.zkoss.zk.au.http.AuDropUploader$StreamVideo@72d28b60
Uploaded: null

Expected Result

after a successful upload the media should not be null
or an error should be displayed indicating the failure

Debug Info

used file size to reproduce was 1.2MB on zkfiddle

Root Cause

Workaround