rack/rack — pull request #451 by pivotal-casebook (2012-11-07 20:43:31+00:00)
Title: Initialize Rack::Sendfile with accel mappings

Instead of requiring the X-Accel-Mapping header to be sent by nginx,
this allows separation of concerns between application and server
configuration.

Example:
config/initializers/rack_sendfile.rb:
```ruby
Example::Application.configure do
config.middleware.swap Rack::Sendfile, Rack::Sendfile,
config.action_dispatch.x_sendfile_header,
Rails.root.join('private', 'uploads') => '/private_uploads'
end
```