Hi 

make server action update manually.

find attachments of that models: 

you have already apply the "documents.mixin"

first step to get folder via _get_document_folder

second step check attachment is not available in folder:

third step If attachment not available in folder, get values "_get_document_vals"

& create the document

Or look below method 

    def _create_document(self, vals):
        """
        Implemented by bridge modules that create new documents if attachments are linked to
        their business models.

        :param vals: the create/write dictionary of ir attachment
        :return True if new documents are created
        """
        # Special case for documents
        if vals.get('res_model') == 'documents.document' and vals.get('res_id'):
            document = self.env['documents.document'].search_fetch(
                [('id', '=', vals['res_id'])], [])
            if document and not document.attachment_id and document.type == 'binary':
                document.attachment_id = self[0].id
            return False

        # Generic case for all other models
        res_model = vals.get('res_model')
        res_id = vals.get('res_id')
        model = self.env.get(res_model)
        if model is not None and res_id and issubclass(self.pool[res_model], self.pool['documents.mixin']):
            vals_list = [
                model.browse(res_id)._get_document_vals(attachment)
                for attachment in self
                if not attachment.res_field and model.browse(res_id)._check_create_documents()
            ]
            vals_list = [vals for vals in vals_list if vals]  # Remove empty values
            self.env['documents.document'].create(vals_list)
            return True
        return False