Hi,

You can try like this,

attachment_id = fields.Many2one('ir.attachment', string="Attachment", required=True)

Thanks

I am not sure I correctly understand your question but attachment stored in ir.attachment model.
There is an easy way to an attachment on any model by the chatter
for that you have to inherit model from mail.thread and add related fields (message_ids) on the view 

https://github.com/odoo/odoo/blob/12.0/addons/event/models/event.py#L81​
https://github.com/odoo/odoo/blob/12.0/addons/event/views/event_views.xml#L273​