Have you considered using the official Ghost JavaScript SDK? With it, you don’t have to worry about content types, formData or blobs. It would be like this:
import GhostAdminAPI from '@tryghost/admin-api'
const token = 'SECRET';
const api = new GhostAdminAPI({
url: 'http://localhost:2368',
key: token,
version: "v5.0",
});
const apiRes = await api.images.upload({file: imagePath});
console.log(apiRes);
It’s documented here: