* Add the Rack::Sendfile middleware
* Make the header to use configurable via config.action_dispatch.x_sendfile_header (default to "X-Sendfile").
* Add Railties tests to confirm that these work
* Remove the :stream, :buffer_size, and :x_senfile default options to send_file
* Change the log subscriber to always say "Sent file"
* Add deprecation warnings for options that are now no-ops
Note that servers can configure this by setting X-Sendfile-Type. Hosting companies and those creating packages of servers specially designed for Rails applications are encouraged to specify this header so that this can work transparently.
