C # determine whether a file is a text file

I checked many pieces of data on the Internet. All the big data files were read as a binary file, and I took a single character segment, which is 255216 for example. jpg. The Code is as follows:

         /// <summary>
        /// Checks the file is textfile or not.
        /// </summary>
        /// <param name="fileName">Name of the file.</param>
        /// <returns></returns>
        public static FileExtension CheckTextFile(string fileName)
        {
            FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
            System.IO.BinaryReader br = new System.IO.BinaryReader(fs);
            string fileType = string.Empty; ;
            try
            {
                byte data = br.ReadByte();
                fileType += data.ToString();
                data = br.ReadByte();
                fileType += data.ToString();
                FileExtension extension;
                try
                {
                    extension = (FileExtension)Enum.Parse(typeof(FileExtension), fileType);
                }
                catch 
                { 

                    extension=FileExtension.VALIDFILE
                }
                return extension;
            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                if (fs != null)
                {
                    fs.Close();
                    br.Close();
                }
            }
        }
    }
    public enum FileExtension
    {
        JPG = 255216,
        GIF = 7173,
        PNG = 13780,
        SWF = 6787,
        RAR = 8297,
        ZIP = 8075,
        _7Z = 55122,
        VALIDFILE=9999999
        // 255216 jpg;  

        // 7173 gif;  

        // 6677 bmp,  

        // 13780 png;  

        // 6787 swf  

        // 7790 exe dll,  

        // 8297 rar  

        // 8075 zip  

        // 55122 7z  

        // 6063 xml  

        // 6033 html  

        // 239187 aspx  

        // 117115 cs  

        // 119105 js  

        // 102100 txt  

        // 255254 sql   

    }  <