使用html5以块的形式上传文件

I am trying to upload a file in chunks using html5's File API and then reassembling it on the server side in php. I am uploading a video but when i merge the files in server side the size has increased and it is becoming a invalid file . Please note the below html5 code only works in browser chrome . Tested in Chrome 9 as this uses file API's slice function . Can some one guide me on this ? Thanks I am trying to upload a file in chunks using ht