Can you post the syncthing logs, as well as the nginx logs?
I assume you've seen this: https://stackoverflow.com/questions/48626459/refused-to-execute-script-because-strict-mime-type-checking-is-enabled
Can you post your nginx config? Is it just this one with different variables? https://docs.syncthing.net/users/reverseproxy.html
I'd definitely take a look at the syncthing logs...