The documentation says:

"If you have uploaded an image as an attachment, they will be automatically generated a thumbnail that will displayed in your message, with an Link to the Full Size Image"

However, though the attachments appear to upload to the attachments page (even though I can only see a small image of them there) in my actual message all I get is something along the lines of:

in otherwords it just shows a tag with what looks like maybe an attachment index.. no thumbnail and no way to view the actual attachment image. I've tried in chrome and edge browsers

Can you check under Admin > Settings > BBCode Extensions if the Attach bbcode extension exist? Or any at all.

If there is no extension then use the import function on that page. You find the extensions in an xml file in the install folder.

You need no manually Install them via Admin > Settings > BBCode Extensions > Import List with the BBCodeExtensions.xml