DMZ Web Proxy and Application server Web

Not sure that it is supported officially, but that is possible, yes. Web servers act as clients to the M-Files server no matter whether they are in DMZ or internal. There is nothing to prevent multiple such clients connecting to one server. I have had a configuration like that for some years now although not using Co-authoring (have no need).

Many thanks

- so just to confirm, what we were looking to do was have external web requests hit the DMZ proxy, and internal web requests be routed to the internal M-Files application server.

Is that what you have in place?

Thanks again

Many thanks

- so just to confirm, what we were looking to do was have external web requests hit the DMZ proxy, and internal web requests be routed to the internal M-Files application server.

Is that what you have in place?

Thanks again

Not 100% the same but close enough to conclude that you can make it work.
I have multiple M-files servers internally. External web traffic is forwarded by my router to the primary internal server, and if the traffic is meant for one of the other servers the primary server will act as a proxy and forward the traffic to the relevant server.
Each of the internal servers have their own web server configured in order to handle internal traffic directly.
Internal DNS points each subdomain directly to its final internal server.
External DNS points all subdomains to the external IP, the router forwards it to the proxy, and the proxy forwards it to the correct server. All M-Files traffic is on gRPC, and it is redirected with URL rewrite.
The only issue I have seen is in mobile access which is only possible to my primary server. It will not redirect that traffic to the other servers.
In your case the proxy will be in DMZ, but that should not make any difference. When you have only one internal M-Files server you should also be able to handle mobile access without issues.