Start a mitmproxy with:
```
mitmproxy -m "reverse:https://YOUR_MATTERMOST_DOMAIN" -H '/~a | ! ~a/Access-Control-Expose-Headers/*, Authorization, Content-Length, Token' -H '/~a | ! ~a/Access-Control-Allow-Origin/http://localhost:8000'
```

Run locally:
```
cd /path/to/repo; python -m "http.serve"
```