Email attachment does not show up in the 'Attachment' Related List Section of the Activity Timeline

The most common root cause of this issue is the 'Enhanced Email' feature being disabled. The email message is added as a completed task in Salesforce where an attachment is not available. To address the issue, the Enhanced Email feature must be enabled.
 

To Enable Enhanced Email

1. Go to Setup
2. Enter Enhanced Email in the Quick Find box.
3. Select Enhanced Email.
4. Click on Enable.

Enhanced Email elevates email to a standard Salesforce object, with customization options and an API. Emails are no longer stored as Tasks, but as full-featured records, complete with details, related lists, and collaboration tools.  For more information, review the Enhanced Email overview.

Review Considerations for Using Enhanced Email before enabling Enhanced Email.