Add `SENDFILE_HEADER` environment variable (#24123)

config/environments/production.rb

@@ -38,8 +38,7 @@ Rails.application.configure do
   # `config.assets.precompile` and `config.assets.version` have moved to config/initializers/assets.rb
 
   # Specifies the header that your server uses for sending files.
-  # config.action_dispatch.x_sendfile_header = 'X-Sendfile' # for Apache
-  config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect' # for NGINX
+  config.action_dispatch.x_sendfile_header = ENV['SENDFILE_HEADER'] if ENV['SENDFILE_HEADER'].present?
 
   # Allow to specify public IP of reverse proxy if it's needed
   config.action_dispatch.trusted_proxies = ENV['TRUSTED_PROXY_IP'].split(/(?:\s*,\s*|\s+)/).map { |item| IPAddr.new(item) } if ENV['TRUSTED_PROXY_IP'].present?