Documentation
Handler: Upload Reporting
The Upload Progress plug-in provides feedback to the web browser about the status of an ongoing POST request.
To make it more convenient, it is implemented so that you don’t have to change a single line of code in your applications. It is compatible with Lighttpd’s mod_uploadprogress, Nginx’s NginxHttpUploadProgressModule, and Apache’s equivalent, so chances are you’ll be able to do a seamless migration.
To make use of the plugin you’ll need some code on the client side. Many Javascript snippets and Flash modules are available for free. The jQuery-upload-progress module has given fairly good results in the past, but there are many other options available.