OkHttp Post MediaType Parse

I am trying to send a single line of text of a employee id in the body of a OkHttp POST request, simulating a request that I see using browser inspection.

The request is a mix of URL params and a single string of employee ID in the body of the post, i.e. ABC123

Currently after login and sending the request the server response is 500.

        Request quickFind = new Request.Builder()
            .url(quickFIndUrl)
            .addHeader("csrf_tok", csrfToken)
            .addHeader("x-rest-api", "true")
            .addHeader("x-requested-with", "true")
            .addHeader("accept-encoding", "gzip, deflate, br")
            .addHeader("user-agent", "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.90 Mobile Safari/537.36")
            .addHeader("accept", "application/json, text/plain, */*")
            .addHeader("origin", "https://contoso.com")
            .addHeader("content-type", "application/json;charset=UTF-8")
            .post(RequestBody.create(MediaType.parse("text/x-markdown"), postBody))
            .build();

What does the line .post(RequestBody.create(MediaType.parse("text/x-markdown"), postBody) do as I think this might be the cause for server response 500.

On face value it looks the like it sets the MIME type, but then how is MIME type different to the content-type above?

What are valid types that can be parsed other than text/x-markdown?