Content-Type header must be set to application/vnd.api+json.
While sending bulk requests, make sure to set both the Content-Type and the Accept header to application/vnd.api+json; ext=bulk:
Content-Type: application/vnd.api+json; ext=bulk
Accept: application/vnd.api+json; ext=bulk
When Content-Type is not set as described, API will return 415 response status error.
When the Accept header is not set as described on a bulk request, the API will return a 404 Route Not Found error, because the bulk route will not be matched.