FIX: 'config.serve_static_files' was renamed to 'config.public_file_server.enabled' in Rails 5.1

config/environments/production.rb

@@ -10,7 +10,7 @@ Discourse::Application.configure do
   config.action_controller.perform_caching = true
 
   # Disable Rails's static asset server (Apache or nginx will already do this)
-  config.serve_static_files = GlobalSetting.serve_static_assets
+  config.public_file_server.enabled = GlobalSetting.serve_static_assets || false
 
   config.assets.js_compressor = :uglifier
 

config/environments/profile.rb

@@ -13,7 +13,7 @@ Discourse::Application.configure do
   config.action_controller.perform_caching = true
 
   # in profile mode we serve static assets
-  config.serve_static_files = true
+  config.public_file_server.enabled = true
 
   # Compress JavaScripts and CSS
   config.assets.compress = true

config/environments/test.rb

@@ -8,7 +8,7 @@ Discourse::Application.configure do
   config.cache_classes = true
 
   # Configure static asset server for tests with Cache-Control for performance
-  config.serve_static_files = true
+  config.public_file_server.enabled = true
 
   # Show full error reports and disable caching
   config.consider_all_requests_local       = true