Fixing Path_traversel when downloading a file

Question

How to fix this code so that it does not accuse this type of vulnerability?

I'm using checkmarx to scan, and he encountered this problem when downloading a file

Error reported by checkmarx:

Method Index at line 13 of src\BR.Rve.UI.Site\Controllers\DownloadFileController.cs gets dynamic data from the fileName element. This element’s value then flows through the code and is eventually used in a file path for local disk access in Index at line 13 of src\BR.Rve.UI.Site\Controllers\DownloadFileController.cs. This may cause a Path Traversal vulnerability." and I'm having a little trouble solving this

and I'm having a little trouble solving this

My original code:

     public FileResult Index(string fileName)
        {
            string rootPath = System.Configuration.ConfigurationManager.AppSettings.Get("FinalUploadFolder");
            byte[] fileBytes = System.IO.File.ReadAllBytes(string.Format("{0}/{1}", rootPath,fileName));            
            return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Octet, fileName);
        }

I've tried this solution below, but it didn't work

      private static readonly char[] InvalidFilenameChars = Path.GetInvalidFileNameChars();
      public ActionResult Index(string fileName)
        {
            if (fileName.IndexOfAny(InvalidFilenameChars) >= 0)
                  return new HttpStatusCodeResult(HttpStatusCode.BadRequest);

            string rootPath = System.Configuration.ConfigurationManager.AppSettings.Get("FinalUploadFolder");
            byte[] fileBytes = System.IO.File.ReadAllBytes(Path.Combine(rootPath,fileName))));            
            return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Octet, fileName);
        }

What could I do in the code to avoid this type of problem when scanning