diff --git a/actionpack/lib/action_controller/caching/pages.rb b/actionpack/lib/action_controller/caching/pages.rb
index a70ed72..598369e 100644
--- a/actionpack/lib/action_controller/caching/pages.rb
+++ b/actionpack/lib/action_controller/caching/pages.rb
@@ -54,7 +54,7 @@ module ActionController #:nodoc:
           @@page_cache_directory = defined?(Rails.public_path) ? Rails.public_path : ""
           cattr_accessor :page_cache_directory
 
-          @@page_cache_extension = '.html'
+          @@page_cache_extension = nil
           cattr_accessor :page_cache_extension
         end
       end
@@ -133,6 +133,10 @@ module ActionController #:nodoc:
       def cache_page(content = nil, options = nil)
         return unless perform_caching && caching_allowed
 
+        should_set_page_cache_extension = !self.class.page_cache_extension
+        if should_set_page_cache_extension
+          self.class.page_cache_extension = ".#{self.request.format.to_sym}" || ".html"
+        end
         path = case options
           when Hash
             url_for(options.merge(:only_path => true, :skip_relative_url_root => true, :format => params[:format]))
@@ -142,7 +146,9 @@ module ActionController #:nodoc:
             request.path
         end
 
-        self.class.cache_page(content || response.body, path)
+        returning(self.class.cache_page(content || response.body, path)) do
+          self.class.page_cache_extension = nil if should_set_page_cache_extension
+        end
       end
 
       private
@@ -151,4 +157,4 @@ module ActionController #:nodoc:
         end
     end
   end
-end
\ No newline at end of file
+end
diff --git a/actionpack/test/controller/caching_test.rb b/actionpack/test/controller/caching_test.rb
index b6cdd11..2df22dc 100644
--- a/actionpack/test/controller/caching_test.rb
+++ b/actionpack/test/controller/caching_test.rb
@@ -89,6 +89,29 @@ class PageCachingTest < Test::Unit::TestCase
     assert File.exist?("#{FILE_STORE_PATH}/index.html")
   end
 
+  def test_should_cache_with_correct_extension
+    xml_format = Mime::Type.lookup('text/xml')
+    @request.stubs(:format).returns(xml_format)
+    @params[:format] = nil
+    get :ok
+    assert_response :ok
+    assert ! File.exist?("#{FILE_STORE_PATH}/page_caching_test/ok.html"),
+      "get with ok status of type text/xml should not have been cached as html"
+    assert File.exist?("#{FILE_STORE_PATH}/page_caching_test/ok.xml"),
+      "get with ok status of type text/xml should have been cached"
+  end
+
+  def test_should_cache_with_default_extension_when_set
+    with_cache_extension('.shtml') do
+      get :ok
+    end
+    assert_response :ok
+    assert ! File.exist?("#{FILE_STORE_PATH}/page_caching_test/ok.html"),
+      "get with ok status with a set page_cache_extension should not have been cached as html"
+    assert File.exist?("#{FILE_STORE_PATH}/page_caching_test/ok.shtml"),
+      "get with ok status with a set page_cache_extension should have been cached"
+  end
+
   def test_should_expire_cache_with_custom_path
     get :custom_path
     assert File.exist?("#{FILE_STORE_PATH}/index.html")
@@ -98,12 +121,16 @@ class PageCachingTest < Test::Unit::TestCase
   end
 
   def test_should_cache_without_trailing_slash_on_url
-    @controller.class.cache_page 'cached content', '/page_caching_test/trailing_slash'
+    with_cache_extension('.html') do
+      @controller.class.cache_page 'cached content', '/page_caching_test/trailing_slash'
+    end
     assert File.exist?("#{FILE_STORE_PATH}/page_caching_test/trailing_slash.html")
   end
 
   def test_should_cache_with_trailing_slash_on_url
-    @controller.class.cache_page 'cached content', '/page_caching_test/trailing_slash/'
+    with_cache_extension('.html') do
+      @controller.class.cache_page 'cached content', '/page_caching_test/trailing_slash/'
+    end
     assert File.exist?("#{FILE_STORE_PATH}/page_caching_test/trailing_slash.html")
   end
 
@@ -146,6 +173,17 @@ class PageCachingTest < Test::Unit::TestCase
     def page_cached?(action)
       File.exist? "#{FILE_STORE_PATH}/page_caching_test/#{action}.html"
     end
+
+    def with_cache_extension(extension)
+      begin
+        prior_extension = ActionController::Base.page_cache_extension
+        ActionController::Base.page_cache_extension = extension
+        yield
+        assert_equal extension, ActionController::Base.page_cache_extension
+      ensure
+        ActionController::Base.page_cache_extension = prior_extension
+      end
+    end
 end
 
 class ActionCachingTestController < ActionController::Base