GZipStream works but extension is lost

I am using following code to zip a file and it works fine but when I decompress with WinRar I get the original file name without the extension, any clue why if filename is myReport.xls when I decompress I get only myReport ?

using (var fs = new FileStream(fileName, FileMode.Open))
{
    byte[] input = new byte[fs.Length];
    fs.Read(input, 0, input.Length);
    fs.Close();

    using (var fsOutput = new FileStream(zipName, FileMode.Create, FileAccess.Write))
    using(var zip = new GZipStream(fsOutput, CompressionMode.Compress))
    {
        zip.Write(input, 0, input.Length);
        zip.Close();
        fsOutput.Close();
    }
}