Use .ref instead of .to_sym.

actionpack/lib/action_controller/log_subscriber.rb

@@ -7,8 +7,10 @@ class LogSubscriber < ActiveSupport::LogSubscriber
     def start_processing(event)
       payload = event.payload
       params  = payload[:params].except(*INTERNAL_PARAMS)
+      format  = payload[:format]
+      format  = format.to_s.upcase if format.is_a?(Symbol)
 
-      info "  Processing by #{payload[:controller]}##{payload[:action]} as #{payload[:formats].first.to_s.upcase}"
+      info "  Processing by #{payload[:controller]}##{payload[:action]} as #{format}"
       info "  Parameters: #{params.inspect}" unless params.empty?
     end
 

actionpack/lib/action_controller/metal/instrumentation.rb

@@ -19,7 +19,7 @@ def process_action(*args)
         :controller => self.class.name,
         :action     => self.action_name,
         :params     => request.filtered_parameters,
-        :formats    => request.formats.map(&:to_sym),
+        :format     => request.format.ref,
         :method     => request.method,
         :path       => (request.fullpath rescue "unknown")
       }