An email attachment index occurs on import. All imaged emails with attachments will contain an index of those attached files in the email header. Changing this setting in your software will only apply to future imports.
To enable the attachment index on all future imports go to SETTINGS > Import.
Scroll down to the find the Attachment Index on Email Images settings box and click "Edit".
To change the attachment index settings, click "Enable Attachment Index".
Below is an example of how the index will display on an imaged email when attachments are present. Shown in the document viewer screen.