How to include private Slack File in Incoming Webhook?

[moroii69]

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I am trying to post a message with a 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:

json
{
  "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: Uploading Private Images with Block Kit

So, my question is:

Is it possible to include a private Slack File in an Incoming Webhook? 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 options, either providing a 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 the image part, it still shows an invalid_blocks error.
• Tried using image_url with a random image on the internet, it works.
• Tried uploading files, both sending it to the channel and not, but still fails.
• Tried giving all permissions for both Bot and User Token, but still fails.
• Tried re-integrating and re-adding the bot to the channel, but still fails.
• Confirm that the bot that sends the File and Message (from Webhook) are the same.