Don't process if the attachment isn't valid.

lib/paperclip/attachment.rb

@@ -105,7 +105,7 @@ module Paperclip
 
       @dirty = true
 
-      post_process(*only_process) if post_processing
+      post_process(*only_process) if post_processing && valid_assignment?
 
       # Reset the file size if the original file was reprocessed.
       instance_write(:file_size,   @queued_for_write[:original].size)
@@ -369,8 +369,14 @@ module Paperclip
       Paperclip.log(message)
     end
 
-    def valid_assignment? file #:nodoc:
-      file.nil? || (file.respond_to?(:original_filename) && file.respond_to?(:content_type))
+    def valid_assignment? #:nodoc:
+      if instance.valid?
+        true
+      else
+        instance.errors.none? do |attr, message|
+          attr.to_s.start_with?(@name.to_s)
+        end
+      end
     end
 
     def initialize_storage #:nodoc:

test/attachment_processing_test.rb

+# encoding: utf-8
+require './test/helper'
+require 'paperclip/attachment'
+
+class AttachmentProcessingTest < Test::Unit::TestCase
+  def setup
+    rebuild_model
+  end
+
+  should 'process attachments given a valid assignment' do
+    file = File.new(fixture_file("5k.png"))
+    Dummy.validates_attachment_content_type :avatar, :content_type => "image/png"
+    instance = Dummy.new
+    attachment = instance.avatar
+    attachment.expects(:post_process)
+
+    attachment.assign(file)
+  end
+
+  should 'not process attachments if the assignment does not pass validation' do
+    file = File.new(fixture_file("5k.png"))
+    Dummy.validates_attachment_content_type :avatar, :content_type => "image/tiff"
+    instance = Dummy.new
+    attachment = instance.avatar
+    attachment.expects(:post_process).never
+
+    attachment.assign(file)
+  end
+end