attachmentInfo is of type
var attachmentInfo = Components.classes["@mozilla.org/messengercompose/attachment;1"].createInstance(Components.interfaces.nsIMsgAttachment);
The attachment is added through an URL from the internet
attachmentInfo.url = uploadLink;
The code works perfectly, but it does not show the attachment window after the attachment is added, so practically the user won’t have any feedback as when the upload completes.
Thank you in advance!