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