Chose to implement the server-side service as a standalone Java app, however using HttpCore and FileUpload libs from Apache Commons to avoid reinventing the wheels of http header and multipart parsers. Also using log4j for logging. I reused some of the example code, there are inline references when I did this.
I took some shortcuts like rudimentary http file serving since the use case didn't really need anything more complex. Also, given that there were not sessions logic beyond supporting various concurrent users I assumed that each user can be identified by their IP to avoid implementing more complex session management. Obviously a better solution would be to have some session generated on the main page request, and keep it across requests.
Used jQuery for javascript, with the google-hosted version, not a FE engineer so it'll surely send shivers down FE engineer's spines. CSS is minimal to show the progress bar.
To execute, run: ant dist ant run
Then navigate to http://localhost/superuploader - this serves public/form.html
Files downloaded go to ./storage, this is configurable from the ant script (ideally could be extracted to a conf file). You'll get output in the server when uploading stuff.
The default port is 80, use ant run -Dport=X to override.