Do not run post_processing if the attachment is not valid.

lib/paperclip/attachment.rb

@@ -83,7 +83,7 @@ module Paperclip
 
       @dirty = true
 
-      post_process
+      post_process if valid?
  
       # Reset the file size if the original file was reprocessed.
       instance_write(:file_size, uploaded_file.size.to_i)

test/attachment_test.rb

@@ -163,6 +163,7 @@ class AttachmentTest < Test::Unit::TestCase
       @attachment.expects(:valid_assignment?).with(@not_file).returns(true)
       @attachment.expects(:queue_existing_for_delete)
       @attachment.expects(:post_process)
+      @attachment.expects(:valid?).returns(true)
       @attachment.expects(:validate)
       @dummy.avatar = @not_file
     end
@@ -192,6 +193,7 @@ class AttachmentTest < Test::Unit::TestCase
       @attachment.expects(:valid_assignment?).with(@not_file).returns(true)
       @attachment.expects(:queue_existing_for_delete)
       @attachment.expects(:post_process)
+      @attachment.expects(:valid?).returns(true)
       @attachment.expects(:validate)
       @dummy.avatar = @not_file
     end

test/integration_test.rb

@@ -209,7 +209,7 @@ class IntegrationTest < Test::Unit::TestCase
       @dummy.avatar = @bad_file
       assert ! @dummy.valid?
       @dummy.avatar = nil
-      assert @dummy.valid?
+      assert @dummy.valid?, @dummy.errors.inspect 
     end
 
     should "know the difference between good files, bad files, not files, and nil when validating" do