Right-click on (or the relevant title). Select Properties > Installed Files (or "Local Files").

If your feature is intended to resolve the "Disk Read Error [zone/code-pre-gfx-mp.ff]" common in older COD titles, the development should focus on :

Fastfiles are tied to specific game patches. If you download a version of code-pre-gfx-mp.ff from an older or newer patch than your current game installation, the game will still crash due to a mismatch.

Never close your PC using the physical power button while the game is running or saving.

If the goal is to view or edit the contents (modding), you would need to develop a parser for the IW FastFile format.

To help you find the exact fix, could you tell me you're playing and what launcher (Steam, Battle.net, or other) you're using?

If you're using custom clients like IW4x or alterIWnet, additional steps may be needed.