If you've played F.E.A.R., these extra missions only offer more of the same old same old.
Extraction Point takes place immediately after the events of F.E.A.R. In fact, it begins with the cinematic that ended the full game. You are the same F.E.A.R operative once again trapped in Auburn trying to find a way out. After slogging through a few levels that really remind you of what you did in the first game, you eventually reach the ending which leaves you in more of a cliffhanger than the original ending.
Perseus Mandate is an alternate mission where you play a member of a second F.E.A.R. squad. It begins before the explosion in Auburn, and ends at about the same time as the Extraction Point mission does. Here you are tasked with finding out more about this Perseus project... which is really just like the same stuff you've been doing all through the original F.E.A.R. and in Extraction Point.
The only real revelation is some added insight into Alma. But again, like the original game, everything is very enigmatic and like an X-Files or Lost episode, leaves you with more questions that you got answers. There are also a handful of instant action maps which are subsets or alterations of levels where you have a set amount of time to clear the levels of baddies. Instant action is just that... full action and shooting, with no story or scares.
A couple benefits of F.E.A.R. Files? It is a stand-alone "game", meaning you don't need to have your F.E.A.R. game or save files on hand anymore. Oh, and the achievements are a little more generous. In all of F.E.A.R., I only got 35 gamerscore total. In Files, I got over 500.