Why dose File.Exists not work for exe's and shortcuts

Question

Why is File.Exists not returning true, no matter what filename I feed into it. Here is my code below

using System;

namespace MyNamespace
{
    public class MainClass
        static void Main()
        {
        string filename = "Firefox";
         if(File.Exists(filename)  ){
         Console.WriteLine("true");
         }
          else{Console.WriteLine("False"); }
        }
    }
}

I tried changing filename variable to a directory, and even a different file name but it still returns false. I'm 100% sure that the files are in the directory