Insert caption to an existing video Youtube API, Error "The request does not contain the caption track contents."

I've been trying to insert a caption into an existing video on youtube.I was able to call the List caption endpoint with no problem. So this is not an Auth issue.

I called the POST https://www.googleapis.com/youtube/v3/captions?part=snippet

with a request body

{"snippet": {"language": "en","name": "English captions","videoId": "MY_VIDEO_ID","isDraft": true   }} 

But I'm getting a "The request does not contain the caption track contents."

https://developers.google.com/youtube/v3/docs/captions/insert

The google api explorer does not show u how to actually attach the caption file on the request.

I tried posting using form data too, but no luck.