Attachment not appearing in Sales Order notification for a particular category

Hi,

I have created a customised document category and assigned it to Sales Order Form. I can view this category in the the form and  I select and attach the file to that Order with new category.When I send the order for approval , WF notification goes to the Supervisor with all the details. The person can see all the details but there is no attyachment displays. When I change the document category to existing category(Prined Documents) then it displays the attachment in the notification. This is behaving very strangely. I am not able to understand why this is happening. Why