NET Core Ok() return json formatted as string in Angular

From .NET Core I return object in Ok() in method with IActionResult result:

 public async Task<IActionResult> UploadJsonFiles()
{
      return Ok(new { status = "Ok", message = "" });
}

In Angular I have that json but it is formatted as string. I don't have object with property status and message, only string:

this.uploader.response.subscribe(res => {
..

res = "{"status":"Ok","message":""}"

In Startup I have set formatters for controllers:

services.AddControllersWithViews().AddNewtonsoftJson();
services.AddControllers(options =>
{
    options.RespectBrowserAcceptHeader = true; // false by default
})
.AddJsonOptions(options =>
        options.JsonSerializerOptions.PropertyNamingPolicy = null);