diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index d696656521..30b643c791 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -82,14 +82,15 @@
 
     *Łukasz Strzałkowski*
 
-*   Fix header `Content-Type: #<Mime::NullType:...>` in localized template.
+*   Fix render of localized templates without an explicit format using wrong
+    content header and not passing correct formats to template due to the
+    introduction of the `NullType` for mimes.
 
-    When localized template has no format in the template name,
-    the response now has the default and correct `content-type`.
+    Templates like `hello.it.erb` were subject to this issue.
 
     Fixes #13064.
 
-    *Angelo Capilleri*
+    *Angelo Capilleri*, *Carlos Antonio da Silva*
 
 *   Try to escape each part of a url correctly when using a redirect route.
 
diff --git a/actionpack/lib/abstract_controller/rendering.rb b/actionpack/lib/abstract_controller/rendering.rb
index 6f17e3fcd9..7be61d94c9 100644
--- a/actionpack/lib/abstract_controller/rendering.rb
+++ b/actionpack/lib/abstract_controller/rendering.rb
@@ -22,7 +22,7 @@ module AbstractController
     def render(*args, &block)
       options = _normalize_render(*args, &block)
       self.response_body = render_to_body(options)
-      _process_format(rendered_format)
+      _process_format(rendered_format) if rendered_format
       self.response_body
     end
 
diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index 66d34f3b67..5c48b4ab98 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -34,8 +34,7 @@ module ActionController
 
     def _process_format(format)
       super
-      # format is a Mime::NullType instance here then this condition can't be changed to `if format`
-      self.content_type ||= format.to_s unless format.nil?
+      self.content_type ||= format.to_s
     end
 
     # Normalize arguments by catching blocks and setting them on :update.
diff --git a/actionpack/lib/action_dispatch/http/mime_negotiation.rb b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
index 346598b6de..c33ba201e1 100644
--- a/actionpack/lib/action_dispatch/http/mime_negotiation.rb
+++ b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
@@ -50,7 +50,7 @@ module ActionDispatch
       #   GET /posts/5       | request.format => Mime::HTML or MIME::JS, or request.accepts.first
       #
       def format(view_path = [])
-        formats.first
+        formats.first || Mime::NullType.instance
       end
 
       def formats
diff --git a/actionpack/lib/action_dispatch/http/mime_type.rb b/actionpack/lib/action_dispatch/http/mime_type.rb
index 2a8ff0a5d2..3d2dd2d632 100644
--- a/actionpack/lib/action_dispatch/http/mime_type.rb
+++ b/actionpack/lib/action_dispatch/http/mime_type.rb
@@ -28,7 +28,7 @@ module Mime
   class << self
     def [](type)
       return type if type.is_a?(Type)
-      Type.lookup_by_extension(type) || NullType.instance
+      Type.lookup_by_extension(type)
     end
 
     def fetch(type)
diff --git a/actionpack/test/controller/send_file_test.rb b/actionpack/test/controller/send_file_test.rb
index 8ecc1c7d73..a4c84c29d7 100644
--- a/actionpack/test/controller/send_file_test.rb
+++ b/actionpack/test/controller/send_file_test.rb
@@ -144,7 +144,7 @@ class SendFileTest < ActionController::TestCase
     }
 
     @controller.headers = {}
-    assert !@controller.send(:send_file_headers!, options)
+    assert_raise(ArgumentError) { @controller.send(:send_file_headers!, options) }
   end
 
   def test_send_file_headers_guess_type_from_extension