File is: TERA-EU\Client\S1Game\Config\S1Input.ini
I made a toggle script so that you can switch between two FOVs (you can make a script change and cycle with more values however). Add these lines to the file anywhere you like after the header [Engine.PlayerInput]. For example, place them under the line that says bEnableMouseSmoothing=true
Code:
Bindings=(Name="SwitchToFov100",Command="fov 100 | SetBind insert SwitchToFov60")
Bindings=(Name="SwitchToFov60",Command="fov 60 | SetBind insert SwitchToFov100")
Bindings=(Name="insert",Command="SwitchToFov100")
The key bound here is "insert", but you can change it to anything you like; for example "PageUp". Replace the word "insert" on all three lines to change it.
Important: To stop the game from overwritting this file you'll need to change its properties to "Read only" after you are done editing. Right click on it -> Properties -> check Read Only.
Finaly, you can modify the FOV value on the command="" section to anything you like. The default the game uses is 60, you may want to try 90 instead of 100 or whatever suits your needs.
As for documentation, general Unreal Engine 3 docs will work. However, not all configuration options found on the files are guaranteed to work in game. As with any game that uses the UE3 engine, or any engine for that matter, it's possible that they have tweaked the base foundation enough so that the exposed externals of the engine aren't even actualy used.
For example, you can find "god mode" tags within the configuration... these will obviously do nothing in game.