Why setting Response.Body makes response content empty

I have an endpoint with the following code:

var range = Request.GetTypedHeaders().Range;
var file = new MemoryStream(Enumerable.Range(0, 255).Select(x => (byte)x).ToArray());
// Response.Body = file;
return File(file, "application/octet-stream", "{{1.txt}}", null, new EntityTagHeaderValue($"\"1.txt\""), true);

And this is how I send request to it:

var request = new HttpRequestMessage { RequestUri = new Uri("http://localhost:5100/file") };
request.Headers.Range = new RangeHeaderValue(0, 100);
var response = new HttpClient().SendAsync(request).Result;
Debug.WriteLine($"ContentLength: {response.Content.Headers.ContentLength}");
Debug.WriteLine(response.RequestMessage);

The print out is:

ContentLength: 100
Method: GET, RequestUri: 'https://localhost:7165/file', Version: 1.1, Content: <null>, 
Headers:
{
  Range: bytes=0-100
}

But if I uncomment line with Response.Body = file content length becomes 0. Why?