IIS Express using an unkown user to perform write

Question

I have an ASP.Net MVC web application in which I am trying to allow the user to upload a PDF file to a folder inside the app.

   string path = "Case_Log_Docs";
            if (!Directory.Exists(path))
            {
                Directory.CreateDirectory(path);
                System.Diagnostics.Debug.WriteLine("Created the folder.");
            }

            if (PostedFile != null)
            {
                string fileName = Path.GetFileName(PostedFile.FileName);
                System.Diagnostics.Debug.WriteLine(fileName);
                PostedFile.CopyTo(new FileStream(path, FileMode.Create));
                ViewBag.Message += string.Format("<b>{0}</b> uploaded.<br />", fileName);
            }

However I am getting an Unauthorized User: Access denied error. I used Process Monitor and monitored this application writing a file and a different (and fully operational) application to do the same thing. I noticed that in the app that is operational IIS Express used myself as the user when attempting (and succeeding) to write the file to the specified folder. However, the the application in question used this value as the user: S-1-5-21-1092590740-2774964217-3234137035-256211 .

I have tried the answers to this question: how to run iisexpress app pool under a different identity

But for the first answer, I do not have a.config file so that doesn't work. I did complete the second answer that directs you to use the runas command but it did not fix my error.

So my 2 questions are:

1.) Why is IIS Express using this value instead of the current user (myself)?

2.) How do I instruct IIS Express to use myself as the user? I was not even aware that IIS Express could use anything other than the current user.