mrXmr , and needs an extenstion for that
Tags, approval and many others are also extensions albeit a first party one.
Fof Upload has received more overhauls and contributions than if it would have been a first party extension, of that i am sure.
I can imagine people being disappointed at their most wanted feature not being part of core, it has honestly been a recurring topic. In the end we simply ignore these, because features are always a matter of opinion and need.
Flarum, or rather the framework (core), excels at enabling any feature and FoF Upload is a perfect example of that. It relies heavily on core code, but it also incenticized changes to it for future extensions (like s3) and itself.
This is never a fun discussion to have, because there is no winner. I still hope to explain why we are so strict in excluding more features as a core responsibility. Our team is small and relies heavily on a two to three people actively pushing Flarum code forward. Adding more responsibilities will move their time away from the roadmap and towards maintenance.
