Hi,

Try the following code

attachment_ids = fields.Many2many('ir.attachment', 'doc_warning_rel', 'doc_id', 'attach_id4', string="Attachment")

def your_custom_function(self):
    attachments = []
    for i in self.attachment_ids:
        attachments.append(i.id)
    self.message_post(body='your message', partner_ids='your partner ids', attachment_ids=attachments)

Regards