Unregister Mime::Type in teardown.

actionpack/test/controller/send_file_test.rb

@@ -32,14 +32,18 @@ class SendFileWithActionControllerLive < SendFileController
 class SendFileTest < ActionController::TestCase
   include TestFileUtils
 
-  Mime::Type.register "image/png", :png unless defined? Mime::PNG
-
   def setup
+    @mime_type_defined = defined? Mime::PNG
+    Mime::Type.register "image/png", :png unless @mime_type_defined
     @controller = SendFileController.new
     @request = ActionController::TestRequest.new
     @response = ActionController::TestResponse.new
   end
 
+  teardown do
+    Mime::Type.unregister :png unless @mime_type_defined
+  end
+
   def test_file_nostream
     @controller.options = { :stream => false }
     response = nil