One possible way is to put nginx as a reverse proxy in between browser and Keycloak server instance. You can dig around using $content_type embedded variable of nginx ngx_http_core_module or may be nginx_rewrite module and a simple tweak (may be an if statement in nginx server/location block config) will help you in achieving the required. Based on the value of content-type header, you can proxy-pass the requests to a different upstream server via nginx.

Cheers,