C# 判断文件真正格式

FileTypeCheck.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.UI.WebControls;

    public static class FileTypeCheck
    {
        //真正判断文件类型的关键函数
        public static bool IsAllowedExtension(HttpPostedFile hifile1,string[] headNumArr)
        {
            HttpPostedFile hifile = hifile1;

            //System.IO.FileStream fs = new System.IO.FileStream(hifile.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);
            System.IO.BinaryReader r = new System.IO.BinaryReader(hifile.InputStream);
            string fileclass = "";
            byte buffer;
            try
            {
                buffer = r.ReadByte();
                fileclass = buffer.ToString();
                buffer = r.ReadByte();
                fileclass += buffer.ToString();
            }
            catch
            { }
            r.Close();
            //fs.Close();

            bool ret = false;

            foreach (string s in headNumArr) {
                if (fileclass == s) {
                    ret = true;
                    break;
                }
            }

            //if (fileclass == "8075")
            //{
            //    MessageBox.Show("xlsx,zip,pptx,mmap,zip");
            //}
            //if (fileclass == "208207")
            //{
            //    MessageBox.Show("xls.doc.ppt");
            //}
            //if (fileclass == "4944")
            //{
            //    MessageBox.Show("csv");
            //}

            //if (fileclass == "255216" || fileclass == "7173")
            ////说明255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar
            //{
            //    return true;
            //}
            //else
            //{
            //    return false;
            //}
            return ret;
        }
    }