diff --git a/actionpack/lib/action_controller/metal/data_streaming.rb b/actionpack/lib/action_controller/metal/data_streaming.rb
index 369741fb129..374645c9cec 100644
--- a/actionpack/lib/action_controller/metal/data_streaming.rb
+++ b/actionpack/lib/action_controller/metal/data_streaming.rb
@@ -8,10 +8,8 @@ module ActionController #:nodoc:
 
     include ActionController::Rendering
 
-    DEFAULT_SEND_FILE_OPTIONS = {
-      :type         => 'application/octet-stream'.freeze,
-      :disposition  => 'attachment'.freeze,
-    }.freeze
+    DEFAULT_SEND_FILE_TYPE        = 'application/octet-stream'.freeze #:nodoc:
+    DEFAULT_SEND_FILE_DISPOSITION = 'attachment'.freeze #:nodoc:
 
     protected
       # Sends the file. This uses a server-appropriate method (such as X-Sendfile)
@@ -127,7 +125,7 @@ module ActionController #:nodoc:
       #
       # See +send_file+ for more information on HTTP Content-* headers and caching.
       def send_data(data, options = {}) #:doc:
-        send_file_headers! options.dup
+        send_file_headers! options
         render options.slice(:status, :content_type).merge(:text => data)
       end
 
@@ -135,15 +133,8 @@ module ActionController #:nodoc:
       def send_file_headers!(options)
         type_provided = options.has_key?(:type)
 
-        options.update(DEFAULT_SEND_FILE_OPTIONS.merge(options))
-        [:type, :disposition].each do |arg|
-          raise ArgumentError, ":#{arg} option required" if options[arg].nil?
-        end
-
-        disposition = options[:disposition]
-        disposition += %(; filename="#{options[:filename]}") if options[:filename]
-
-        content_type = options[:type]
+        content_type = options.fetch(:type, DEFAULT_SEND_FILE_TYPE)
+        raise ArgumentError, ":type option required" if content_type.nil?
 
         if content_type.is_a?(Symbol)
           extension = Mime[content_type]
@@ -157,10 +148,13 @@ module ActionController #:nodoc:
           self.content_type = content_type
         end
 
-        headers.merge!(
-          'Content-Disposition'       => disposition,
-          'Content-Transfer-Encoding' => 'binary'
-        )
+        disposition = options.fetch(:disposition, DEFAULT_SEND_FILE_DISPOSITION)
+        unless disposition.nil?
+          disposition += %(; filename="#{options[:filename]}") if options[:filename]
+          headers['Content-Disposition'] = disposition
+        end
+
+        headers['Content-Transfer-Encoding'] = 'binary'
 
         response.sending_file = true
 
diff --git a/actionpack/test/controller/send_file_test.rb b/actionpack/test/controller/send_file_test.rb
index 3af17f495cb..6fc3556e319 100644
--- a/actionpack/test/controller/send_file_test.rb
+++ b/actionpack/test/controller/send_file_test.rb
@@ -154,6 +154,17 @@ class SendFileTest < ActionController::TestCase
     end
   end
 
+  def test_send_file_with_default_content_disposition_header
+    process('data')
+    assert_equal 'attachment', @controller.headers['Content-Disposition']
+  end
+
+  def test_send_file_without_content_disposition_header
+    @controller.options = {:disposition => nil}
+    process('data')
+    assert_nil @controller.headers['Content-Disposition']
+  end
+
   %w(file data).each do |method|
     define_method "test_send_#{method}_status" do
       @controller.options = { :stream => false, :status => 500 }