Did some digging of my own in the browser's debugger (I know a small amount of programming but I'm no dev, not by a long-shot), and I found something that may be an issue.

It appears to me that lines 52-56 were meant to be commented out, but are not. I'm not sure if this is intentional, or an oversight. Hopefully this helps.

Additional error details:

I've had the same error as above. I used the console to get this error message:

I'm running Windows 10, browser is Firefox 46.0.1

Some quick Google-Fu says this is some sort of JavaScript error.

This error only happens sometimes, but it does occur more often than I am able to successfully upload.