3 files changed, 28 insertions, 2 deletions
diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 48ba1518e0..cc042b5ac3 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,5 +1,10 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Added `NullMimeTypeObject` class. This  allows to use html?, xml?, json?..etc when
+    the `format` of `request` is unknown.
+   
+    *Angelo  Capilleri*
+
 *   `date_select` helper accepts `with_css_classes: true` to add css classes similar with type
     of generated select tags.
 
diff --git a/actionpack/lib/action_dispatch/http/mime_type.rb b/actionpack/lib/action_dispatch/http/mime_type.rb
index 3d560518e1..5ee6f2056e 100644
--- a/actionpack/lib/action_dispatch/http/mime_type.rb
+++ b/actionpack/lib/action_dispatch/http/mime_type.rb
@@ -153,7 +153,7 @@ module Mime
       end
 
       def lookup_by_extension(extension)
-        EXTENSION_LOOKUP[extension.to_s]
+        EXTENSION_LOOKUP[extension.to_s] || NullMimeTypeObject.new
       end
 
       # Registers an alias that's not used on mime type lookup, but can be referenced directly. Especially useful for
@@ -301,6 +301,17 @@ module Mime
         method.to_s.ends_with? '?'
       end
   end
+
+  class NullMimeTypeObject
+    private
+    def method_missing(method, *args)
+      if method.to_s.ends_with? '?'
+        false
+      else
+        super
+      end
+    end
+  end
 end
 
 require 'action_dispatch/http/mime_types'
diff --git a/actionpack/test/dispatch/request_test.rb b/actionpack/test/dispatch/request_test.rb
index e2964f9071..b3466faffb 100644
--- a/actionpack/test/dispatch/request_test.rb
+++ b/actionpack/test/dispatch/request_test.rb
@@ -590,7 +590,17 @@ class RequestTest < ActiveSupport::TestCase
 
     request = stub_request
     request.expects(:parameters).at_least_once.returns({ :format => :unknown })
-    assert request.formats.empty?
+    assert_instance_of Mime::NullMimeTypeObject , request.format
+  end
+
+
+  test "format is not nil with unknown format" do
+    request = stub_request
+    request.expects(:parameters).at_least_once.returns({ format: :hello })
+    assert_equal request.format.html?, false
+    assert_equal request.format.xml?, false
+    assert_equal request.format.json?, false
+    assert !request.format.html?
   end
 
   test "formats with xhr request" do