Unable to use send mail attachments callback on Service Desk project

The Script Listener "Send custom email" allows users to set a custom callback to filter the attachments.
If the listener is mapped on a Service Desk project, the following custom callback

import com.onresolve.scriptrunner.canned.jira.workflow.postfunctions. 
{
    MailAttachment a -> a.isNew()
}

will not return any attachment because the isNew() method always return a false value even when there is an attachment on the new comment made.

Steps to replicate:

• Create a "Send a custom email" listener mapped to a Service Desk project
• Map it to an event like "Issue Commented"
• Select "HTML" as mail format
• On the "Include Attachments" field select "Custom"
• Place this script inside the "Custom attachment callback" field:

  import com.onresolve.scriptrunner.canned.jira.workflow.postfunctions.MailAttachment
  {
      MailAttachment a -> a.isNew()
  }
  

• Now create an issue from the Service Desk portal
• Add a comment with an attachment
• The mail does not contain the full attachment file but only the standard Service Desk link to the attachment