Attachment to the email template not getting sent when multiple invoices are sent

Attachment to the email template not getting sent when multiple invoices are sent

I have an attachment in invoice notification email template (please see the screenshot). It is a newsletter that I'd like to send to all customers with invoices. But when I select multiple invoices and click email icon button to send them it sends emails without that newsletter attachment. But if I select only one invoice and click the email button it sends the attachment. Is this a bug?
Thanks and regards
Kush