Yep, sounds about right. Other extensions can also handle upload, but FoF upload is the most popular one.
I'm assuming that by 'the flarum cloud', you mean Free Flarum. One thing to note is that this is not an official flarum service, but rather the result of a community members generosity. There isn't a technical limitation of the flarum software that would cause issues with FoF upload, rather that allowing uploads can result in security issues and expensive hard drive costs.
