Changed the default content_type validation message to something more relevant.

lib/paperclip.rb

@@ -190,7 +190,7 @@ module Paperclip
           unless options[:content_type].blank?
             content_type = instance[:"#{name}_content_type"]
             unless valid_types.any?{|t| t === content_type }
-              options[:message] || ActiveRecord::Errors.default_error_messages[:inclusion]
+              options[:message] || "is not one of the allowed file types."
             end
           end
         end