Using the example from the Elastic Search Tutorial:
$ curl -XPOST 'http://localhost:5000/index/blog/post/1' -d '
{
    "user": "dilbert",
    "postDate": "2011-12-15",
    "body": "Search is hard. Search should be easy." ,
    "title": "On search"
}' -H 'Content-Type: application/json'
Just add the -H option to explicitly pass the content type as an HTTP header.