Like all various versions of Ransomware, there are many different versions of Teslacrypt that can be easily modified to target specific file types (or in this case, game files) as its pretty much a toolkit you buy on less than legit sites. Its also reported that its a heavily modded cryptolocker which makes tools that can unencrypt the files useless.
The biggest distributor of it was an unnamed wordpress site that dropped it in via a flash exploit. In my opinion, chances are that you're more likely to get this through a steam message from an infected user (an account on your friend's list messaging you out of the blue going 'LOL look at this *insert fake image site URL here*'). That way the distributor of Teslacrypt can be more efficient as Steam is targeted (oddly not Uplay or Origin..). Obviously as time goes on, it'll be bought and modified countless times.