Script Listener does not trigger sometimes on Issue Commented

[ys]

We're trying to create a simple Helpdesk workflow with basic Jira (not Service Desk).

Our current setup uses the generic mail handler to "Create new issue or add comment", and we have a "Fast Track transition" listener act upon comments if current status == "To Do", the transition will move the ticket to another state

I noticed that if the email being sent contains only text, then the listener will work and the transition triggers properly. HOWEVER, if the email contains an attachment (or even small images within the email, 5~10kb in size), then the listener does not trigger at all.  That particular event does not even show up in the execution history of the listener, even though the comment created from the email appears in the ticket itself. 

Anyone know what could cause this?