Attaching report data to email

1. Write the XLS to the output directory as you would do normally, then take the content of the file as a byte array and delete the file from the ouput directory.
2. Your mail content is a MimeMultipart, that can hold both your mail text as well as your attachment.

Example code for getting the byte array:

wb.write();
var excelFileByteArray = Context.getFile(chosenFilename, Constants.LOCATION_OUTPUT);
Context.deleteFile(chosenFilename, Constants.LOCATION_OUTPUT);

Example code for the mail (this is for xlsx - take the appropriate MIME type for the ByteArrayDataSource from https://learn.microsoft.com/de-de/archive/blogs/vsofficedeveloper/office-2007-file-format-mime-types-for-http-content-s…):

this.send = function(sContent, additionalRecipients, subject, attachmentByteArray) {
    
    // set up your java.util.Properties here
    
    var instSession = Packages.javax.mail.Session.getInstance(props, null);
    var smtpTransport = instSession.getTransport("smtp");
     
    // Receiver Address:
    var msg = new Packages.javax.mail.internet.MimeMessage(instSession);
    msg.setFrom(adrFrom);
     
    // insert proper date
    msg.setSentDate(new Date());
    
    
    var recipients = [new Packages.javax.mail.internet.InternetAddress("redacted2@example.com")];
    recipients = recipients.concat(additionalRecipients);
    msg.setRecipients(Packages.javax.mail.internet.MimeMessage.RecipientType.TO, recipients);
     
    // "Subject": Subject of the mail    
    msg.setSubject(subject, "UTF-8");
    
    //main body
    var multipart = new Packages.javax.mail.internet.MimeMultipart();
    
    //message body - sContent is a JavaScript string containing HTML
    var messagePart = new Packages.javax.mail.internet.MimeBodyPart();
    messagePart.setContent(sContent, "text/html");
    multipart.addBodyPart(messagePart);
    
    //message attachment https://stackoverflow.com/a/23084334
    var attachmentPart = new Packages.javax.mail.internet.MimeBodyPart();
    var byteArrayDataSource = new Packages.javax.mail.util.ByteArrayDataSource(attachmentByteArray, "application/vnd.openxmlformatsofficedocument.spreadsheetml.sheet");
    attachmentPart.setDataHandler(new Packages.javax.activation.DataHandler(byteArrayDataSource));
    attachmentPart.setFileName(chosenFilename);
    multipart.addBodyPart(attachmentPart);

    msg.setContent(multipart);
    
    smtpTransport.connect();
    smtpTransport.send(msg);
    smtpTransport.close();
}