The basic media type for the API is application/vnd.api+json. If you want to execute a request that includes data (POST, PATCH, DELETE) you should include this media type in the Content-Type header. The only case where a different Content-Type header will be accepted is for endpoints that can accept file like objects where the media type used should be multipart/form-data.
The base media type this API responds is application/vnd.api+json. To retrieve the data in this media type you should include it in the Accept header. The only case where the server will return another media type is for endpoint that return file like objects. In this case the media type used should be */* and the server will prioritize the negotiation to the media type dictated from the file. Note that application/vnd.api+json will be also available for Accept header in these cases and will return a JSON serialized representation of the file.
Read more here: https://jsonapi.org/format/#content-negotiation