Image included with forum reply not rendering when included in email notification

Whether you see placeholders for images or no images at all, depends on the browser you're using and it’s configured settings.

I checked into this and noticed that images are not missing in all email notifications.

Mail notifications with missing user avatar and site/group logo

These are notifications for new replies to forum posts, new comments to blog posts, etc.
I suspect that these notifications are sent by Zimbra/Telligent Job Service.
If you check the actual mail message in your mail client, you’ll notice that it only contains one block of mime-encoded (base64) content.
There are no mime-encoded images for the user’s avatar and site/group logo.
You can check this instantly in GMail: there is no paperclip icon to the right of the message’s subject.

mail notifications with inline mime-encoded user avatar and site/group logo

Mail notifications that do show the user’s avatar and the site/group logo, are probably not sent by Zimbra/Telligent Job Service.
In GMail, they have a paperclip icon to the right of the message subject.
I assume they are sent by the Zimbra Community application itself.
These are welcome mails to new users, invitations for joining a group, etc.
But also the Sent Preview Emails for testing template changes in the admin Control Panel.
You can easily test this yourself.
Enter the Control Panel and select Site Administation => Site Configuration => Manage Plugins.
Filter the plugins by selecting “Notifications” from the filter selection box.
Scroll down to “Forum Reply Notifications” and click button "Configure" to the right.
In the "Forum Reply Notifications" pop-up window, select tab “Templates”.
At the bottom of the window, click button “Send Preview Email”.
You will now receive a sample notification email with the group/site logo image shown.
This mail is sent to the email address in your Zimbra administrator account.
If you check the actual mail message, by selecting “Show original” in the message actions menu in GMail, you will see the group/site logo mime-encoded within the message itself.
This mime-encoded group/site logo starts with lines:

    ---=_ANM001_=-ad7877c43a83451b863573f6283216e1
    Content-Transfer-Encoding: base64
    Content-ID: <sitelogo-image-250x88.png-150x42x2.png@yourdomain.com>
    Content-Type: image/png; name=sitelogo-image-250x88.png-150x42x2.png.png


I have checked numerous settings in the Control Panel and configuration files.
I could not find any setting that controls including of mime-encoded images in mail notifications.
Could this be a bug??