using System;
using System.IO;
using System.Linq;

public class Test
{
    public static void Main()
    {
        string ContentType = "application/octet-stream";
        string FileName = "test.xls";
        bool validate = false;

        string[] AllowedTypes = { "application/vnd.ms-excel",
                                  "application/ms-excel",
                                  "application/x-msexcel",
                                  "application/excel",
                                  "application/x-excel",
                                  "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
                                };

        //Para o fallback, acaso o navegador enviar a extensão octet-stream ou zip
        string[] FallckExtensions = { ".xls", ".xlsx" };
        string[] FallbackMimes = { "application/octet-stream", "application/zip",
                                    "application/x-zip-compressed" };

        //Se ContentType for um dos FallbackMimes, então checa a extensão
        if (FallbackMimes.Contains(ContentType)) {
            string extension = Path.GetExtension(FileName).ToLower();

            validate = FallckExtensions.Contains(extension);
        } else {
            //Se o ContentType enviado pelo navegador não for octet-stream ou zip testa o AllowedTypes
            validate = AllowedTypes.Contains(ContentType);
        }

        Console.Write(validate ? "Validou" : "Não validou");
    }
}