I am using Google Apps Script (GAS) to upload an image file to Slack via the files.completeUploadExternal API method. Below is the response object that I receive:

{
  "ok": true,
  "files": [
    {
      "id": "F072RT14KME",
      "created": 1714969813,
      "timestamp": 1714969813,
    }
  ]
}

However, neither the created nor the timestamp fields from the response could be used as thread_ts for replying to the uploaded message.

Question:
How can I send an image file to Slack using the API and reply to that message using thread_ts?

Additional Information:

I am using Google Apps Script for this purpose.