First,this text is translated by DeepL.
Forgive me if you feel uncomfortable.

The programming language used is gas.

I want to upload video files stored in GoogleDrive to Slack.
I would like to send sonmeone a DM.

Please help me!

This is my GAS code.

function myFunction() {
  let token = "Slack_Bot_User_Outh_Token"
  let bot_token = "Slack_User_Outh_Token"
  let user_id = "slack_user_id"

  let user_channel_original_id = get_user_channel_id(token, user_id)
  let file_info = DriveApp.getFileById("Drive_file_id")
  let slack_file_uploaded_id = get_upload_url(token, file_info)
  post_slack_to_person(token, slack_file_uploaded_id, user_channel_original_id)
}

function get_user_channel_id(token, user_id) {
  let slack_conversion_url = "https://slack.com/api/conversations.open"
  let options = {
    "method": "post",
    "payload": {
      "token": token,
      "users": user_id
    }
  }
  let res = UrlFetchApp.fetch(slack_conversion_url, options)
  let user_channel_id = JSON.parse(res.getContentText()).channel.id
  return user_channel_id
}

function get_upload_url(token, file_info) {
  let send_toslack_url = "https://slack.com/api/files.getUploadURLExternal"

  let options = {
    "method": "get",
    "payload": {
      "token": token,
      "filename": file_info,
      "length": file_info.getSize().toString(),
    }
  }
  let res = UrlFetchApp.fetch(send_toslack_url, options)
  return JSON.parse(res).file_id
}

function post_slack_to_person(token, file_id, channel_id) {
  let post_directmessages = "https://slack.com/api/files.completeUploadExternal"

  let options = {
    "method": "post",
    "headers": {
      "Authorization": "Bearer " + token,
    },
    "contentType": "application/json",
    "payload": JSON.stringify(
      {
        files: [
          {
            "id": file_id,
          }
        ],
        "channel_id": channel_id,
      },
    )
  }
  let res = UrlFetchApp.fetch(post_directmessages, options)
  Logger.log(res)
  return res
}

And this is response.

{
  "ok": true, "files": [
    {
      "id": "I can't show you",
      "created": 1716643818,
      "timestamp": 1716643818,
      "name": "TEST.mp4",
      "title": "TEST.mp4",
      "mimetype": "",
      "filetype": "",
      "pretty_type": "",
      "user": "I can't show you",
      "user_team": "I can't show you",
      "editable": false,
      "size": 13659221,
      "mode": "hosted",
      "is_external": false,
      "external_type": "",
      "is_public": false,
      "public_url_shared": false,
      "display_as_bot": false,
      "username": "",
      "url_private": "I can't show you",
      "url_private_download": "I can't show you",
      "media_display_type": "unknown",
      "permalink": "I can't show you",
      "permalink_public": "I can't show you",
      "comments_count": 0,
      "is_starred": false,
      "shares": {},
      "channels": [],
      "groups": [],
      "ims": [],
      "has_more_shares": false,
      "has_rich_preview": false,
      "file_access": "visible"
    }
  ],
  "warning": "missing_charset",
  "response_metadata": {
    "warnings": [
      "missing_charset"
    ]
  }
}

Documentation: https://api.slack.com/methods/files.completeUploadExternal https://api.slack.com/methods/files.getUploadURLExternal