判断文件类型（文件的真正类型，不是根据扩展名判断）,通过文件头来判断

                bool xx=false;  //default sFileName is not Exe or Dll File
                System.IO.FileStream  fs=new System.IO.FileStream(sFileName,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)
                {
                    Console.WriteLine(exc.Message);
                }
                r.Close();
                fs.Close();
                if (bx=="7790"||bx=="8297"||bx=="8075")//7790:exe,8297:rar,8075:pk
                {
                    xx=true;
                }
                Console.WriteLine(bx);
                return xx;

                dll:MZ
                exe:MZ
                rar:Rar
                zip:PK