upload file control should not accept any exe and dll files

if (!IsExeFile(ReadAllBytes(postedFile.FileName)))                      
                        {
                           extension = System.IO.Path.GetExtension(postedFile.FileName).ToLower();
                         
                            if (ContentTypeList.Contains(extension) != null || extension != string.Empty)
                            {
                               
                                mimeType = ContentTypeList.Contains(extension).MimeType;

                                if (mimeType != string.Empty)
                                {
                                    maxLength = Framework.Global.ContentTypeList.Contains(extension).Size;
                                    uploadFile = true;
                                }
                            }
                        }

  private static byte[] SubByteArray(byte[] data, int index, int length)
        {
            byte[] result = new byte[length];
           
            Array.Copy(data, index, result, 0, length);
           
            return result;
        }
      
  
   public static bool IsExeFile(byte[] FileContent)
        {
            var twoBytes = SubByteArray(FileContent, 0, 2);
            return ((Encoding.UTF8.GetString(twoBytes) == "MZ") || (Encoding.UTF8.GetString(twoBytes) == "ZM"));
        }