Showing a file upload progress bar

Uploading files from the web browser to the server is a rather common feature of modern web applications. Any number of CMS or publishing systems allows users to upload images to include these with their textual content, as shown in the following image:

Here is an example of a web upload in progress. The basic idea behind one of the algorithms to implement the progress bar is to initiate a POST request in an IFrame and then poll some well-known URL for the progress counter. Modern browsers allow us to get the progress information right on the client's side; this is a part of XMLHttpRequest Level 2 and was standardized about 3 years ago. There are a lot of older web applications that still rely on the older methods.

The described method only works if your client-side posts to your server-side with the same speed that the user actually sees in their interface. The problem is Nginx that buffers the long POST and then quickly and efficiently pushes it to the...