Extjs: XML Tag Name Mismatch

I'm using FileUploadField.js in Extjs. It was working, but then I changed the URL to which I was submitting the form, and then started getting the following error:

XML tag name mismatch (expected br)

This was curious, and I couldn't figure out what the issue was. I googled and found very little help on the subject.

The solution? The problem was in my back-end server code. There was a null pointer exception in my Java code, which returned an HTML error page that is standard for our app. The Extjs processing was expecting JSON, and couldn't find it, obviously. Hence, the error.

If you are using Firebug (which you should be, everyone should be), you will be able to trace the error better by looking at the "Net" tab when you access your Extjs component that is throwing the error. After that, it was an easy fix.