Update FileCallbackResult.cs

[natalie-o-perret]

A smol' MR changing the stream passed to the callback:

• from the "Response Body Stream"
• to the "Response Body Writer property as a stream"
  as the former could trigger "Synchronous operations are disallowed" exceptions with Kestrel (and one doesn't necessarily wanna enable that support).

[brunomartinspro]

Thanks! this workaround avoided the

 var syncIOFeature = HttpContext.Features.Get<IHttpBodyControlFeature>();
 
 if (syncIOFeature != null)
 {
     syncIOFeature.AllowSynchronousIO = true;
 }