Revert "compute ext in initialize, and use an attr_reader"

This reverts commit 2dbb73bd.

Conflicts:

	actionpack/lib/action_dispatch/middleware/static.rb

actionpack/lib/action_dispatch/middleware/static.rb

@@ -2,16 +2,11 @@
 
 module ActionDispatch
   class FileHandler
-    attr_reader :ext
-
     def initialize(at, root)
       @at, @root     = at.chomp('/'), root.chomp('/')
       @compiled_at   = /^#{Regexp.escape(at)}/ unless @at.blank?
       @compiled_root = /^#{Regexp.escape(root)}/
       @file_server   = ::Rack::File.new(@root)
-
-      ext = ::ActionController::Base.page_cache_extension
-      @ext = "{,#{ext},/index#{ext}}"
     end
 
     def match?(path)
@@ -32,6 +27,13 @@ def match?(path)
     def call(env)
       @file_server.call(env)
     end
+
+    def ext
+      @ext ||= begin
+        ext = ::ActionController::Base.page_cache_extension
+        "{,#{ext},/index#{ext}}"
+      end
+    end
   end
 
   class Static