data.cs

private void MailMerge_IncludeTextMerging(object sender,
    TXTextControl.DocumentServer.MailMerge.IncludeTextMergingEventArgs e)
{
    // custom handing of XLSX files
    switch (Path.GetExtension(e.IncludeTextField.Filename))
    {
        case ".xlsx": // Excel file detected

            if (!File.Exists(e.IncludeTextField.Filename))
                return;

            // load document into temp. ServerTextControl
            using (TXTextControl.ServerTextControl tx =
                new TXTextControl.ServerTextControl())
            {
                tx.Create();

                // Bookmark name is the sheet name of the Excel document
                TXTextControl.LoadSettings ls = new TXTextControl.LoadSettings()
                {
                    DocumentPartName = e.IncludeTextField.Bookmark
                };

                try
                {
                    // load the Excel document
                    tx.Load(
                        e.IncludeTextField.Filename, 
                        TXTextControl.StreamType.SpreadsheetML, 
                        ls);
                }
                catch {
                    return;
                }

                byte[] data;

                // save the document using the TX format
                tx.Save(out data, 
                    TXTextControl.BinaryStreamType.InternalUnicodeFormat);

                e.IncludeTextDocument = data; // pass back to the field
            }

            break;
    }
}