Putting attachments on invoices

I need to attach photo evidence of receipts to invoices. I capture them using the CB app, and they appear in the Bills Inbox from where I transfer them to the File Store. When I have drafted the invoice, I view it on Income>Invoices and attach the attachments at Options>Manage>Attachments for the invoice in question - I click upload and find the files to be attached in the File Store or elsewhere. I then check the invoice and press send - however the invoice is arriving with the client, but not the attachments. What am I doing wrong? Thanks