The latest version of attachment_fu will give you this error. You need to replace a line in this file:

vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:386

+ errors.add attr_name, I18n.translate('activerecord.errors.messages.inclusion') unless enum.nil? || enum.include?(send(attr_name))    
- errors.add attr_name, ActiveRecord::Errors.default_error_messages[:inclusion] unless enum.nil? || enum.include?(send(attr_name))