How to customize function compress by zip to accept extensio

Question

I work on c# app doing compress to file using ziparchieve 

my dot net version 4.7 

CreateZipFile(string fileName, out List files, extension type xlsx)

I need to pass extension type as optional parameter and if no extension type passed then select all files on folder

if extension type as xlsx passed then compress files that have extension xlsx only

also I need list of string files as out because may be I used again

public static void CreateZipFile(string fileName, List files)
{
    using (ZipArchive zip = ZipFile.Open(fileName, ZipArchiveMode.Update))
    {
        foreach (var file in files)
        {
            if (!zip.Entries.Any(zipFile => file.Contains(zipFile.FullName)))
            {
                zip.CreateEntryFromFile(file, Path.GetFileName(file), CompressionLevel.Optimal);
            }
        }
    }
}