Multi authors of upload. If mod was made by 5 people, all of them would be authors of uploaded file.

I don't really know how someone could abuse such a feature. The worst thing you could do is declaring people as authors that were not involved (this could be fixed with a simple confirmation system). Creating multi-accounts to specify them as authors makes absolutely no sense... but well... some retards will do it for sure. You're probably right.

I still don't plan to implement it soon because the current system was not designed for it and I don't think it's worth the work. I assume that it is sufficient - for now - to name co-authors in the file description (using the user:-tag). And of course to let the most active user of all involved users upload the thing so the user can be contacted by the other ones in order to change the file if necessary.

Would be great if someone comments on our uploaded files and the author receive the notifications like friend request.

I agree with this. Same for replies in threads you made or in threads/files where you posted yourself. The idea is not a new one though. I'll add something for this sooner or later.

Something like Facebook notifications, right? It'll be nice to make 'follow post' function. You get notification from thread you follow.

Not a bad idea, but, when you type "@", a pop-up with the users who have posted in that thread is listed + a search function.
Isn't that enough yet?

Date & time of the latest download of a file. I would like to know when lastly my or other user's file has been downloaded.