Send email with attachment using Javascript for automation

I would like to use Automation Javascript in OS X Yosemite to create a new email message in Mail.app and attach a file to the email. This is my code:

Mail = Application('com.apple.Mail')
message = Mail.OutgoingMessage().make()
message.visible = true
message.toRecipients.push(Mail.Recipient({ address: "abc@example.com" }))
message.subject = "Test subject"
message.content = "Lorem ipsum dolor sit"

      

        
        
        
      

    

It works great up to this point. I see a new message box with correctly filled in recipient, subject and body text. But I can't figure out how to add the file attachment to the post. The scripting dictionary for Mail.app indicates that the property contents

(instance RichText

) can contain attachments, but I don't know how to add it.

I tried this, but I get the error:

// This doesn't work.
attachment = Mail.Attachment({ fileName: "/Users/myname/Desktop/test.pdf" })
message.content.attachments = [ attachment ]
// Error: Can't convert types.

      

        
        
        
      

    

I found some examples on the internet how to do this in AppleScript, like this one :

tell application "Mail"
    ...
    set theAttachmentFile to "Macintosh HD:Users:moligaloo:Downloads:attachment.pdf"
    set msg to make new outgoing message with properties {subject: theSubject, content: theContent, visible:true}

    tell msg to make new attachment with properties {file name:theAttachmentFile as alias}
end tell

      

        
        
        
      

    

But I can't figure out how to convert this to Javascript.