Validate media type spoofing only when attachment changed.

It resulted with downloading attachment (i.e. from S3) on each
instance validation even when attachment didn't change at all.

lib/paperclip/has_attached_file.rb

@@ -79,7 +79,8 @@ module Paperclip
     end
 
     def add_required_validations
-      @klass.validates_media_type_spoof_detection @name
+      name = @name
+      @klass.validates_media_type_spoof_detection name, :if => ->{ send(name).dirty? }
     end
 
     def add_active_record_callbacks

test/validators/media_type_spoof_detection_validator_test.rb

@@ -18,8 +18,6 @@ class MediaTypeSpoofDetectionValidatorTest < Test::Unit::TestCase
 
   should "return default error message for spoofed media type" do
     build_validator
-
-    # Make avatar dirty
     file = File.new(fixture_file("5k.png"), "rb")
     @dummy.avatar.assign(file)
 
@@ -29,4 +27,21 @@ class MediaTypeSpoofDetectionValidatorTest < Test::Unit::TestCase
 
     assert_equal "has an extension that does not match its contents", @dummy.errors[:avatar].first
   end
+
+  should "run when attachment is dirty" do
+    build_validator
+    file = File.new(fixture_file("5k.png"), "rb")
+    @dummy.avatar.assign(file)
+    Paperclip::MediaTypeSpoofDetector.stubs(:using).returns(stub(:spoofed? => false))
+
+    @dummy.valid?
+
+    assert_received(Paperclip::MediaTypeSpoofDetector, :using){|e| e.once }
+  end
+
+  should "not run when attachment is not dirty" do
+    Paperclip::MediaTypeSpoofDetector.stubs(:using).never
+    @dummy.valid?
+    assert_received(Paperclip::MediaTypeSpoofDetector, :using){|e| e.never }
+  end
 end