Thumbnail upload is not working

We are using 7.x-6.4 version of Brightcove. While the upload of videos to Brightcove is working, we are not able to upload the poster or thumbnail image along with the Video. We don't see any error messages and Brightcove is returning a 200 code. But the thumbnail/poster is not set in the uploaded video. Is this a bug or are we missing something?
I had enabled the logging inside the Brightcove API library and here is the trace of API call made to Brightcove:

array (
  'request' => 'POST https://ingest.api.brightcove.com/v1/accounts/xyz/videos/abc/ingest-requests',
  'request_body' => '{"master":{"url":"https:\\/\\/www.example.com\\/sites\\/default\\/files\\/test_thumb_01_2.mp4"},"profile":"screencast","poster":{"url":"https:\\/\\www.example.com\\/sites\\/default\\/files\\/488285f8-poster.jpg","width":640,"height":362},"thumbnail":{"url":"https:\\/\\/www.example.com\\/sites\\/default\\/files\\/488285f8-thumb.jpg","width":480,"height":272},"capture-images":false}',
  'headers' =>
  array (
    0 => 'Authorization: Bearer AGdxA***_8Q',
    1 => 'Content-Length: 414',
    2 => 'Content-Type: application/json',
  ),
  'response' =>
  array (
    0 => 200,
    1 => '{"id":"77428206-38a0-48d8-8624-4f38a2dbaedd"}
',
  ),
  'response_headers' => 'HTTP/1.1 200 OK
Access-Control-Allow-Headers: Content-Type, Authorization, Content-Length, X-Requested-With, Cache-Control
Access-Control-Allow-Methods: GET,PUT,POST,PATCH,DELETE,OPTIONS
Access-Control-Allow-Origin: *
BCOV-instance: i-03c96a32, fd392b2, 2016-09-13 10:24:41.273Z
BCOV-REQUEST-ID: 761c2b57-fe36-451a-b2bc-6fa9423a0762
Content-Type: application/json; charset=UTF-8
Date: Tue, 13 Sep 2016 10:24:41 GMT
Server: nginx
Strict-Transport-Security: max-age=600
X-Originating-URL: https://ingest.api.brightcove.com/v1/accounts/xyz/videos/abc/ingest-requests
Content-Length: 46
Connection: keep-alive

',
)