const response = await fetch(base_url + "/uploads/" +
  encodeURIComponent(course.upload.upload_id) + "/parts/" + part_number, {
  method: "PUT",
  headers: {
    Authorization: authorization,
    "Content-Type": "application/octet-stream",
    "Content-Length": String(bytes.length)
  },
  body: bytes
});
const receipt = await response.json();
if (!response.ok) throw new Error(receipt.error);
// { ok: true, part_number: 1 }