You can do this. You need to override the "action_invoice_sent" method and create ir.attachment record by passing the value of you binary field and then pass the attachment_id default to attachment_ids field of the composer.

@api.multi
def action_invoice_sent(self):
    res = super(Invoice, self).action_invoice_sent()
    ctx = res.get('context')
    # create ir.attachment record using your binary field
    ctx.update({'default_attachment_ids': [attachment_id]})
    return res