Sending attachments to ms teams from an external bot framework

I tried using the api, when I send only text message, it is working fine, when I add the attachment payload as mentioned in the documentation https://learn.microsoft.com/en-us/microsoftteams/platform/resources/bot-v3/bots-files

{
  "attachments": [{
    "contentType": "application/vnd.microsoft.teams.file.download.info",
    "contentUrl": "https://contoso.sharepoint.com/personal/johnadams_contoso_com/Documents/Applications/file_example.txt",
    "name": "file_example.txt",
    "content": {
      "downloadUrl" : "https://download.link",
      "uniqueId": "1150D938-8870-4044-9F2C-5BBDEBA70C9D",
      "fileType": "txt",
      "etag": "123"
    }
  }]
}

, I am facing the issue the issue 

{
    "error": {
        "code": "BadArgument",
        "message": "Unknown attachment type"
    }
}

the api I used to send messages to the msteams is https://smba.trafficmanager.net/amer/v3/conversations/a****/activities

I just wanted to clarify, in the payload download url is the url which we get by the PUT API for 

https://graph.microsoft.com/v1.0/drives/:driveId/root:/doc1/sample3.txt:/content

and passing attachment as a binary file, in the reesponse I am getting