tested using .net core 3.1

streaming file through C#.cs

// controller 

        public async Task DownloadFile([FromQuery] DownloadFileQueryRequest request, CancellationToken cancellationToken)
        {
            var responseObj = await process(request, cancellationToken);

            HttpContext.Response.ContentType = MediaTypeNames.Application.Octet;
            HttpContext.Response.StatusCode = StatusCodes.Status200OK;
            HttpContext.Response.Headers["Access-Control-Expose-Headers"] = "Content-Disposition";
            HttpContext.Response.Headers["Content-Disposition"] = new ContentDispositionHeaderValue("attachment")
            {
                FileName = $"Subject.{request.filterDate:yyyy-MM-dd}.GeneratedAt.{DateTime.UtcNow:yyyy-MM-ddTHH-mm-ss}.json.gz"
            }
            .ToString();

            await using var gzipStream = new GZipStream(HttpContext.Response.Body, CompressionMode.Compress);

            await JsonSerializer.SerializeAsync(gzipStream, responseObj, cancellationToken: cancellationToken);
        }
    
 // integration test

    // Assert 

    ...
     var fileNamePattern = "^Subject." + $"{filterDate:yyyy-MM-dd}"
                + @".GeneratedAt.(?<GeneratedAt>\d{4}-\d{2}-\d{2}T\d{2}-\d{2}-\d{2}).json.gz$";

    // Act

    var (httpResponseMessage, contentStream) = await _setup.GetStream(url);
    await using var contentStreamDecompressed = new GZipStream(contentStream, CompressionMode.Decompress);
    var responseResult = await JsonSerializer.DeserializeAsync<ResponseObjType>(contentStreamDecompressed);
    var responseFileName = httpResponseMessage.Content.Headers.ContentDisposition.FileName;

    // Assert

    httpResponseMessage.StatusCode.Should().Be(HttpStatusCode.OK);
    responseResult.Should().BeEquivalentTo(responseObjExpected);
    responseFileName.Should().MatchRegex(fileNamePattern);

    var generatedAtText = new Regex(fileNamePattern)
              .Match(responseFileName)
              .Groups["GeneratedAt"]
              .Value;

    var generatedAt = DateTime.ParseExact(generatedAtText, "yyyy-MM-ddTHH-mm-ss", null);

    generatedAt.Should().BeCloseTo(DateTime.UtcNow, precision: TimeSpan.FromMinutes(1));


 // setup test
 
     public async Task<(HttpResponseMessage, Stream)> GetStream(string url, string accept = null, IReadOnlyDictionary<string, string> headers = null)
    {
        var request = new HttpRequestMessage(HttpMethod.Get, url);

        if (accept != null)
            request.Headers.Add("Accept", accept);

        if (headers != null)
            foreach (var (key, value) in headers)
                request.Headers.Add(key, value);

        var result = await _client.SendAsync(request);

        return (result, await result.Content.ReadAsStreamAsync());
    }