文件编码格式收集（亲自试验过）

        public static Encoding GetFileEncoding(byte[] content)
        {
            if (content.Length > 0)
            {
                switch (content[0])
                {
                    case 104:
                        return Encoding.Default;
                    case 255:
                        //有UTF-16,UTF-32                       
                        return Encoding.Unicode;
                    case 254:
                        return Encoding.BigEndianUnicode;
                    case 239:
                        return Encoding.UTF8;
                    case 43:
                        return Encoding.UTF7;
                    case 167:
                        //繁体下的ANSI
                        return Encoding.GetEncoding("BIG5");
                    case 196:
                        //简体下的ANSI
                        return Encoding.GetEncoding("GB2312");
                    default:
                        return Encoding.Default;
                }
            }
            return Encoding.Default;
        }