Not able to get custom attribute data from Autodesk BIM360 for data management API

Not able to get custom attribute data from Autodesk BIM360 for data management API – forge.autodesk.com/en/docs/data/v2/reference/http/projects-project_id-folders-folder_id-contents-GET/

When I was trying to get all data available in a specific project with a specific folder, but I didn’t get the custom attribute data in that. I also need custom attribute data for the file included in that folder. Whereas I am able to get only data and included values with missing custom attribute values. Also in the API custom attribute values are missing. please find it below.

data": [
    {
      "type": "folders",
      "id": "urn:adsk.wipprod:dm.folder:hC6k4hndRWaeIVhIjvHu8w",
      "attributes": {
        "name": "Plans",
        "displayName": "Plans",
        "createTime": "2015-11-27T11:11:23.000Z",
        "createUserId": "BW9RM76WZBGL",
        "createUserName": "John Doe",
        "lastModifiedTime": "2015-11-27T11:11:27.000Z",
        "lastModifiedTimeRollup": "2015-11-27T11:11:27.000Z",
        "lastModifiedUserId": "BW9RM76WZBGL",
        "lastModifiedUserName": "John Doe",
        "path": "/dm-test-root/f0cb4ba0-7722-0133-9814-0eeb7bad1e3b",
        "objectCount": 4,
        "hidden": false,
        "extension": {
          "type": "folders:autodesk.bim360:Folder",
          "version": "1.0",
          "schema": {
            "href": "/schema/v1/versions/folders%3Aautodesk.bim360%3AFolder-1.0"
          },
          "data": {
            "allowedTypes": [
              "folders",
              "items:autodesk.bim360:File",
              "items:autodesk.bim360:Document",
              "items:autodesk.bim360:TitleBlock"
            ],
            "visibleTypes": [
              "folders",
              "items:autodesk.bim360:Document"
            ],
            "namingStandardIds": []
          }
        }

I would like to save custom attributes with their values also.please the image of custom attribute with their values in format of rows and colums