Unable to insert images and Videos, and attach files

I have been trying to insert images and videos and attach files to contents, using API v3. But, I have not been able to do that. Here are some example JSON objects I am sending.

When I am sending the JSON object for inserting images and videos, the POST request seems to work fine. However, the images and videos that I had put into the JSON doesn't seem to be posted, while the content is posted. Example JSON for images and videos that I have been using -

{
  "content":{
      "text":"description","type":"text/html"
  },
"contentImages":[{
  "ref":"https://www.lamborghini.com/en-en/sites/en-en/files/DAM/lamborghini/model/one-off/centenario/gallery/thumb/centenario-1-thumb.jpg",
  "height":201,"contentType":"image/jpeg",
  "type":"image"
  }],
"authorship":"open",
"subject":"Pushpak's test discussion 009",
"contentVideos":[{
  "height":201,"videoSource":"https://www.youtube.com/watch?v=708mjaHTwKc","width":251
  }],
"parent":"/api/core/v3/places/myPlaceId",
"question":true,
"type":"discussion"
}

In the above JSON the text seems to go through, while the images and videos are not posted. I get 201 as the response to this request.

When I am trying to attach files from my computer or from web, I am either getting an error:- unable to fetch fie. Here is the JSON that I am sending for this request

{
  "content":{
      "text":"description","type":"text/html"
  },
"authorship":"open",
"subject":"Pushpak's test discussion 009",
"attachments": [ {
      "doUpload": true,
  "url":"https://i.ytimg.com/vi/3vPo1c2f1I8/maxresdefault.jpg"
  } ],
"parent":"/api/core/v3/contents",
"question":true,
"type":"discussion"
}