Most files downloaded from the Tactical Ops Archive are in 7-zip .7z format for optimal compression of files.
It's free opensource software downloadable from 7-zip.org
When installing custom files its always recommended to make a separate installation for offline use only.
This is to avoid file mismatch errors when joining multiplayer servers.
Where to place files
.u, .ini and .int files -> TO-Version/System
.unr files -> TO-Version/Maps
.utx files -> TO-Version/Textures
.uax files -> TO-Version/Sounds
.umx files -> TO-Version/Music
If you are having trouble installing certain files make sure to check for an included readme, often times it will have instructions on how to install and use it.