wpForo Embeds is not a file attachment addon. Please read the addon description under the price section. This addon is a 3rd party content embedding addon, like YouTube, Vimeo, SoundCloud, Twitter, etc… For example, this addon allows embedding a YouTube video URL and see the video player in the post content once it's submitted. This addon has nothing to do with file uploading and attachment.

If you need an advanced file attachments features, you should check out the wpForo Advanced Attachments Addon: https://gvectors.com/product/wpforo-advanced-attachments/

I don't know why image embeds is not working. For example I have an image attachment here that is not displayed. It is just showing as an attachment

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