How to write chunked content in ASP.NET Core - asp.net-core

Question

I have a simple controller in ASP.NET Core that proxies images from the web and provides a resized version when query parameters are passed.
Some of the source images do not have a Content-Length and Transfer-Encoding: chunked, but...
The base class FileResultExecutorBase does not set the Transfer-Encoding header, even though the ContentLength can be null. My understanding is, that this header is necessary when Content-Length is not provided.
When I set the header manually it is ignored.
My sample code:
class Controller
{
public async Task<IActionResult> Proxy(string url)
{
var response = await httpClient.GetAsync(url);
return new FileCallbackResult("", (stream, context) =>
{
await using (var sourceStream = await response.Content.ReadAsStreamAsync())
{
await sourceStream.CopyToAsync(stream);
}
};
}
}
public delegate Task FileCallback(Stream body, HttpContext httpContext);
public sealed class FileCallbackResult : FileResult
{
public FileCallback Callback { get; }
public FileCallbackResult(string contentType, FileCallback callback)
: base(contentType)
{
Callback = callback;
}
public override Task ExecuteResultAsync(ActionContext context)
{
var executor = context.HttpContext.RequestServices.GetRequiredService<FileCallbackResultExecutor>();
return executor.ExecuteAsync(context, this);
}
}
public sealed class FileCallbackResultExecutor : FileResultExecutorBase
{
public async Task ExecuteAsync(ActionContext context, FileCallbackResult result)
{
var (_, _, serveBody) = SetHeadersAndLog(context, result, result.FileSize, result.FileSize.HasValue);
if (result.FileSize == null)
{
context.HttpContext.Response.Headers[HeaderNames.TransferEncoding] = "chunked";
}
if (serveBody)
{
var bytesRange = new BytesRange(range?.From, range?.To);
await result.Callback(
context.HttpContext.Response.Body,
context.HttpContext);
}
}
}
It works fine locally, but I deploy it to our kubernetes cluster with istio, the requests cannot be served. I guess that istio or another proxy does not like the header combination. I have not found anything in the logs yet.