How to include private Slack File in Incoming Webhook?

I am trying to post a message with private Slack File using Incoming Webhook.

Steps taken :

1. Setup Slack App Incoming Webhook including its permissions and channel integration.
2. Upload File using steps mentioned here.
3. Trigger Webhook using Postman.
4. Got invalid_blocks with HTTP error 400 code.

Request Body :

{
  "text": "Danny Torrence left a 1 star review for your property.",
  "blocks": [
    {
      "type": "section",
      "text": {
        "type": "mrkdwn",
        "text": "Danny Torrence left the following review for your property:"
      }
    },
    {
      "type": "section",
      "block_id": "section567",
      "text": {
        "type": "mrkdwn",
        "text": "<https://example.com|Overlook Hotel> \n :star: \n Doors had too many axe holes, guest in room 237 was far too rowdy, whole place felt stuck in the 1920s."
      },
      "accessory": {
        "type": "image",
        "image_url": "https://is5-ssl.mzstatic.com/image/thumb/Purple3/v4/d3/72/5c/d3725c8f-c642-5d69-1904-aa36e4297885/source/256x256bb.jpg",
        "alt_text": "Haunted hotel image"
      }
    },
    {
      "type": "section",
      "block_id": "section789",
      "fields": [
        {
          "type": "mrkdwn",
          "text": "*Average Rating*\n1.0"
        }
      ]
    },
    {
      "type": "image",
      "title": {
        "type": "plain_text",
        "text": "Please enjoy this photo of a kitten"
      },
      "block_id": "image4",
      "slack_file": {
        "url": "https://files.slack.com/files-pri/00000000000-00000000000/testphoto.jpg"
      },
      "alt_text": "An incredibly cute kitten."
    }
  ]
}

One thing I haven't tried is using sharedPublicURL API, however what I am expecting is not to share the file publicly.

I found an article from Slack related to what I expect : https://slack.com/blog/developers/uploading-private-images-blockkit

so, my question is :

1. Is it possible to include private Slack File in Incoming Webhook?
2. If yes, could you guide me on what steps that I missed from above?

Quite new to Slack App Integration, if you need more info, please tell me. Thank you.

Here is what I have tried :

• Use both option, either providing url or id here.
• Validate Request Body using Block Kit Builder, and it shows fine (tried sending it to slack and it works as well).
• Tried removing all Request Body, except image part, it still shows invalid_blocks error.
• Tried using image_url with random image on the internet, it works.
• Tried uploading files, both sending it to channel and not, but still fails.
• Tried giving all permissions for both Bot and User Token, but still fails.
• Tried re-integrate and re-add bot to channel, but still fails.
• Confirm bot that sends the File and Message (from Webhook) are the same.

After days of research and testing, seems Incoming Webhooks App (bot) doesn't have the permission to access the file my bot has uploaded (I used the same bot when uploading file and for triggering Webhook). I even tried to upload the file to the bot itself by sharing it to the bot Channel ID, but still fails.

My assumption is based on Slack's article above :

As such, ensure that the user (or app) that is posting blocks containing secure image elements has access to the underlying file. For example, say your app uploads a file using a user token (i.e. acts on behalf of a specific user) but posts a message with a secure image element leveraging that file using a bot token (i.e. posts a message as the app), or vice versa. In this situation, the user who uploaded the file did not give explicit permissions to the app to view the file! Try to use the same token to upload files as well as display messages containing them.

I decided to find an alternative by using chat.postMessage API here and now it's working well.