How can I view an attachment in an email that is within a thread?

Issue
An email is showing the attachment icon on the document list but opening the email and expanding the Threads section, this show a ''No available attachments''.

Cause
This is because the way the email was originally created and filed (including the attachments) as a thread.

Solution
The only way to preview these attachments is using Export (refer to Exporting Documents) and to look at the attachments in the downloaded email.