attachment entry using a jco

Hi,

I need to attach a document to an invoice.

using IAttachment2, i am able to move the files to the directory where all the attachments exists,but I don't see the attachments from the Invoice.

This is because, I don't see an attachmententry option for a document.

How can i make a relation between an invoice and attachments using a jco?

Thanks

Ramakanth