Send mail with attachment using mailjet in GAS - email

Question

I am trying to send mail with a PDF attachment using Mailjet from Google Apps Script. I can send regular mail without issue, but not quite sure how to include an attachment.
Mailjet documentation: https://dev.mailjet.com/guides/#sending-with-attached-files
I think my problem is the Base64Content. I don't really understand how to encode the attachment. When I execute the below code, an email is being sent with a PDF attachment, but when I try and open the PDF it displays: "Cannot display PDF (test.pdf is of invalid format)".
var newDoc = DocumentApp.openById(newDocid)
var attachment = newDoc.getAs(MimeType.PDF);
var mailjeturl = "https://api.mailjet.com/v3.1/send";
var mailjetparams = {
"Messages":[{
"From": {"Email": 'myemail#domain.com',"Name": 'Robert'},
"To": [{"Email": 'theiremail#domain.com'}],
"Subject": 'subject',
"HTMLPart": 'this message',
"Attachments": [{"ContentType": "application/pdf",
"Filename": "test.pdf",
"Base64Content": Utilities.base64Encode(attachment)}]
}]
}
var mailjetoptions = {
'method': 'post',
'contentType': 'application/json',
'payload': JSON.stringify(mailjetparams),
'headers': {'Authorization': 'Basic ' + Utilities.base64Encode(MAILJET_KEY)}
};
var response = JSON.parse(UrlFetchApp.fetch(mailjeturl, mailjetoptions))

Answer 1

Try to encode like this:
var pdf = newDoc.getAs('application/pdf').getBytes();
var attach = {fileName:'test.pdf',content:pdf, mimeType:'application/pdf'};
var mailjetparams = {
"Messages":[{
"From": {"Email": 'sender#domain.com',"Name": 'Robert'},
"To": [{"Email": 'reciever#domain.com'}],
"Subject": 'subject',
"HTMLPart": 'this message',
"Attachments": [attach]
}]
}

Answer 2

How about the following modification? I thought that the reason of the error may be Utilities.base64Encode(attachment).
Modification points :
attachment retrieved by newDoc.getAs(MimeType.PDF) is Blob. The Blob cannot be directly converted to base64 using Utilities.base64Encode(). So please convert Blob to byte array, and convert to base64.
When this is reflected to your script. The modification part is as follows.
From :
var attachment = newDoc.getAs(MimeType.PDF);
To :
var attachment = newDoc.getAs(MimeType.PDF).getBytes();
or
var attachment = newDoc.getBlob().getBytes();
When Google Document is retrieved as a Blob, Google converts automatically to PDF. You can also use this.
Reference :
base64Encode()
If I misunderstand your question, I'm sorry.