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();
}

Perfect, thanks Kay, I have this working brilliantly now!

The javax.mail package appear to have been removed from the most recent releases, with little notice. 

Also, it seems to choke on the TLSv2 authentication supported by Office 365 (or at least I haven't found any way of making it work): having a wide-open smtp relay is a bit dangerous these days.