An external service application discovery process that connects a host with an external application server running a service application allowing a user of a client computing device to work with a file via a web browser. The host brokers the functionality of an external application server and serves as the platform where interactions between the client computing device and the external application server occur. An open interface protocol, which is a shared communication protocol, allows the host to communicate instructions from the client computing device to the external application server. Through the external service application discovery process, the external application server describes functionality provided by the service application to the host. The host selectively makes the functionality provided by the service application available to users based on the level of implementation of the open platform interface and the conventions of the external service application discovery process understood by the host.