# I solved this problem this way:

from base64 import b64encode

data = file.read()

self.models.execute_kw(
	db,
	uid,
	password,
	"ir.attachment",
	"create",
	[
		{
			"res_model": model_name,
			"res_id": model_id,
			"type": "binary",
			"name": name,
			"display_name": name,
			"mimetype": mimetype,
			"datas": b64encode(data).decode("utf-8"),
		},
	],
)