Looks like they don’t use TLS at all by default, just MTProto on port 443 or MTProto over HTTP. Comms to the telegram servers are always encrypted with MTProto, but tunneling MTProto over TLS would make any attacks on MTProto much harder (perhaps impossible) to execute.

I thought they used TLS wrapping in some markets for censorship resistance, but apparently that is not the case unless you set up your own proxy.