Low-level web-server interface that is the basis of all Ruby web frameworks. A Rack server is anything that responds to `call(env)` and returns a 3-element array (status, headers, body). Supports socket hijacking to escape the request/response cycle for persistent connections such as WebSockets. Shrine is built on Rack rather than Rails so it can be used from any Ruby web framework, including its upload endpoint which can be mounted in any router.