I've created a test thread on my forums, where I uploaded five images and inserted them all as thumbnails. On my site, some thumbnails show up, but some show up as attachments. They are all .png files. When I do the same test with the same images here at XF, they all show up as thumbnails (as they should).
I see 2 images in the body of the message, and one as a separate attachment. If an attachment isn't referenced in the body, it will be shown below. If you edit the contents of that post, you should only see 2 attachments in the actual post.
The only way to really get more information would be to see a video of the post being created, as from what I see, I don't see anything wrong.
I've overwrote my files, disabled XFOptimize (figuring that *might* be the issue). Tried changing my ./js back to local (from CDN). Still, no luck. The other Add-Ons I have seem harmless and don't affect other sites with exact same configuration. (MailChimp, ragtek New User Conversation, TapaTalk, XenUtiles)