Determines the default Content-Type header which will be transmitted with the outbound HTTP request.
If the parent UnityWebRequest does not have a custom Content-Type header set, then the value of this property will be used to determine the value of the Content-Type header for the HTTP request.
Note: If the parent UnityWebRequest has a custom Content-Type header set, then the value of this property is ignored. If this property is null or an empty string, and the parent UnityWebRequest has no custom Content-Type header, then a default Content-Type will be assigned (usually application/octet-stream).
Default value: null.