读取文件真实格式

获取文件的真实类型，参考下面代码。但不知为何，zip和office2007的文件获取到得一样。

  private string CheckTrueFileName(string fileName)
        {

            System.IO.FileStream fs = new System.IO.FileStream(fileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);
            System.IO.BinaryReader r = new System.IO.BinaryReader(fs);
            string bx = " ";
            byte buffer;
            try
            {
                buffer = r.ReadByte();
                bx = buffer.ToString();
                buffer = r.ReadByte();
                bx += buffer.ToString();
            }
            catch (Exception exc)
            {
                MessageBox.Show("读取文件格式有误！");
                return "";
            }
            r.Close();
            fs.Close();

            //string a = System.IO.Path.GetFileNameWithoutExtension(fileName);
            ////文件格式
          // string b=System.IO.Path.GetExtension(fileName);
            return bx;
        }

 

 public enum FileExtension
    {
        JPG = 255216,
        GIF = 7173,
        BMP = 6677,
        PNG = 13780,
        DLL = 7790,//COM,EXE
        RAR = 8297,
        ZIP = 8075,//xlsx,zip,pptx,mmap,DOCX,epub
        XML = 6063,
        HTML = 6033,
        ASPX = 239187,
        CS = 117115,
        JS = 119105,
        TXT = 210187,
        SQL = 255254,
        BAT = 64101,
        BTSEED = 10056,
        RDP = 255254,
        PSD = 5666,
        PDF = 3780,
        CHM = 7384,
        LOG = 70105,
        REG = 8269,
        HLP = 6395,
        DOC = 208207,//XLS,ppt
    }