How to send files over WebSocket with websocat tool?

I brought a wss:// server up using websocat:

websocat -E -t -v --pkcs12-der=q.pkcs12 wss-listen:0.0.0.0:8443 mirror:

And on the client I am running this command to establish a secure WebSocket connection:

websocat  wss://:8443

I can send text using this setup and whatever I send gets echoed back. But now I want to send files on the wss:// connection. How do I do that? This is how I am sending files using ws:// and it's working fine. Server:

websocat -s -v 0.0.0.0:8765 > received.txt

Client:

websocat -b ws://:8765 < testfile.txt

I tried something like this: Server:

websocat -E -t -v --pkcs12-der=q.pkcs12 wss-listen:0.0.0.0:8443 writefile:received.txt

Client:

websocat -E -t wss://echo.websocket.org:8443 < testfile.txt

But I am getting this error:

websocat: WebSocketError: I/O failure
websocat: error running

Any help regarding how do I do it with wss:// connection will really help. Thanks. If it's not achievable using websocat, then what other tool I can use to bring up a wss:// server and send files to it?