I had uploaded one file(simple_flask.py) to be used as server to the ticket when I created the ticket. Since ticket allows only one file upload, I'd pasted relevant code in ticket itself. I've now uploaded the client python file(flask_client.py) to ticket which should help you reproduce this. This is not using any third party library.

Also regarding content-length - It was part triaging I was doing, I continue to get the issue even if I remove that header completely. You can notice in sample client program that I have attached.