Are there any 'icons' in the email? Little graphics of any sort?
That symbol isn't part of e-mail message; it was added by your e-mail reader because message supposedly contains attachments. Have you looked at original message in AR System Email Messages (or ticket's WorkInfo/WorkLog) form? If an attachment was supposed to be sent, you should be able to find it there; it is also possible that you're using template which presumes that an attachment will be added and structures message to include it, but it isn't added for some reason.
You can also check below points additionally :
- AR System Email Messages whether it has attachment or not.
- Check AR System Email Error Messages form
- User has permission to that form and attachment fields
- Capture filter and escalation log on the event of send mail
I would also check the template code used in this email. It looks like custom HTML template used in this notification.