Microsoft stack Json serializers are pretty much obsolete and should be avoided at all costs. Instead you should be using (and newer .NET Web stack already is by default) Json.NET implementation.
If you don't have it installed you can do so by running Install-Package Newtonsoft.Json
in your NuGet console window. Also make sure you are using Newtonsoft.Json;
.
[WebMethod]
[ScriptMethod(UseHttpGet = true,ResponseFormat = ResponseFormat.Json)]
public static string CurrentFile()
{
var d = new { FileID = "123" };
return JsonConvert.SerializeObject(d);
}