Attachments to invoices not visible online at the PAY NOW link

Attachments to invoices not visible online at the PAY NOW link

I have the various options checked to include the attachments to my invoices when they are viewed online.  However, when my invoice is sent to a client that is not using the Client Portal the attachments are not showing up.  What happens is that:
  1. the client clicks on the "PAY NOW" link in the email that is sent out with the invoice
  2. and then they click on the invoice number at the top of the Pay Now webpage
  3. their invoice is then displayed, but without the attachments... even though they should be there according to my settings
However, if the same client logs into their Client Portal, and then views the same invoice, the attachment are suddenly properly displayed.  Why is it that a client needs to log into the Portal before they can see their attachments?  Should the client not be able to see the attachments as well at the PAY NOW screen?  Many of my clients will probably not use the Portal, so I need them to see the attachments at the PAY NOW webpage.  Why are the attachments not displaying there?