Skip post processing on the size matcher

Fixes #646

lib/paperclip/matchers/validate_attachment_size_matcher.rb

@@ -67,9 +67,12 @@ module Paperclip
           override_method(file, :size){ new_size }
           override_method(file, :to_tempfile){ file }
 
+          @subject.send(@attachment_name).post_processing = false
           @subject.send(@attachment_name).assign(file)
           @subject.valid?
           @subject.errors[:"#{@attachment_name}_file_size"].blank?
+        ensure
+          @subject.send(@attachment_name).post_processing = true
         end
 
         def lower_than_low?

test/matchers/validate_attachment_size_matcher_test.rb

@@ -68,5 +68,19 @@ class ValidateAttachmentSizeMatcherTest < Test::Unit::TestCase
         assert_rejects @matcher, @dummy
       end
     end
+
+    context "post processing" do
+      setup do
+        @dummy_class.validates_attachment_size :avatar, :greater_than => 1024
+
+        @dummy = @dummy_class.new
+        @matcher = self.class.validate_attachment_size(:avatar).greater_than(1024)
+      end
+
+      should "be skipped" do
+        @dummy.avatar.expects(:post_process).never
+        assert_accepts @matcher, @dummy
+      end
+    end
   end
 end