diff --git a/actionpack/lib/action_view/base.rb b/actionpack/lib/action_view/base.rb
index ae6b284854d99..fe51af62e6607 100644
--- a/actionpack/lib/action_view/base.rb
+++ b/actionpack/lib/action_view/base.rb
@@ -379,6 +379,12 @@ def assign_variables_from_controller
         @assigns.each { |key, value| instance_variable_set("@#{key}", value) }
       end
 
+      def set_controller_content_type(content_type)
+        if controller.respond_to?(:response)
+          controller.response.content_type ||= content_type
+        end
+      end
+
       def execute(method, local_assigns = {})
         send(method, local_assigns) do |*names|
           instance_variable_get "@content_for_#{names.first || 'layout'}"
diff --git a/actionpack/lib/action_view/renderable.rb b/actionpack/lib/action_view/renderable.rb
index 4f865cbced296..2b825ac4e943d 100644
--- a/actionpack/lib/action_view/renderable.rb
+++ b/actionpack/lib/action_view/renderable.rb
@@ -24,10 +24,13 @@ def compiled_source
     memoize :compiled_source
 
     def render(view, local_assigns = {})
+      compile(local_assigns)
+
       view._first_render ||= self
       view._last_render = self
+
       view.send(:evaluate_assigns)
-      compile(local_assigns)
+      view.send(:set_controller_content_type, mime_type) if respond_to?(:mime_type)
       view.send(:execute, method(local_assigns), local_assigns)
     end
 
diff --git a/actionpack/lib/action_view/template.rb b/actionpack/lib/action_view/template.rb
index eba42518d7803..1f528dd9009bd 100644
--- a/actionpack/lib/action_view/template.rb
+++ b/actionpack/lib/action_view/template.rb
@@ -22,6 +22,11 @@ def format_and_extension
     end
     memoize :format_and_extension
 
+    def mime_type
+      Mime::Type.lookup_by_extension(format) if format
+    end
+    memoize :mime_type
+
     def path
       [base_path, [name, format, extension].compact.join('.')].compact.join('/')
     end
diff --git a/actionpack/lib/action_view/template_handlers/builder.rb b/actionpack/lib/action_view/template_handlers/builder.rb
index 335ec1abb40e1..7d24a5c423a94 100644
--- a/actionpack/lib/action_view/template_handlers/builder.rb
+++ b/actionpack/lib/action_view/template_handlers/builder.rb
@@ -6,8 +6,7 @@ class Builder < TemplateHandler
       include Compilable
 
       def compile(template)
-        # ActionMailer does not have a response
-        "controller.respond_to?(:response) && controller.response.content_type ||= Mime::XML;" +
+        "set_controller_content_type(Mime::XML);" +
           "xml = ::Builder::XmlMarkup.new(:indent => 2);" +
           "self.output_buffer = xml.target!;" +
           template.source +
diff --git a/actionpack/test/controller/render_test.rb b/actionpack/test/controller/render_test.rb
index 041c54c7fde38..76832f571335c 100644
--- a/actionpack/test/controller/render_test.rb
+++ b/actionpack/test/controller/render_test.rb
@@ -197,11 +197,11 @@ def default_render
 
   def render_alternate_default
     # For this test, the method "default_render" is overridden:
-    @alternate_default_render = lambda {
-	render :update do |page|
-	  page.replace :foo, :partial => 'partial'
-	end
-      }
+    @alternate_default_render = lambda do
+      render :update do |page|
+        page.replace :foo, :partial => 'partial'
+      end
+    end
   end
 
   def rescue_action(e) raise end
@@ -467,6 +467,11 @@ def test_should_render_xml_but_keep_custom_content_type
     get :render_xml_with_custom_content_type
     assert_equal "application/atomsvc+xml", @response.content_type
   end
+
+  def test_should_use_implicit_content_type
+    get :implicit_content_type, :format => 'atom'
+    assert_equal Mime::ATOM, @response.content_type
+  end
 end
 
 class EtagRenderTest < Test::Unit::TestCase
diff --git a/actionpack/test/fixtures/test/implicit_content_type.atom.builder b/actionpack/test/fixtures/test/implicit_content_type.atom.builder
new file mode 100644
index 0000000000000..2fcb32d2473a3
--- /dev/null
+++ b/actionpack/test/fixtures/test/implicit_content_type.atom.builder
@@ -0,0 +1,2 @@
+xml.atom do
+end