A lot of people who bought the DLC can't connect to the servers and get kicked leaving them with this error: "Files are different from server".
The only way to actually play the game, is to remove the DLC. Create a subfolder on your desktop or delete these files.
This is what you do.
Make sure your game is closed.
1. Go into your steam folder. Click on steam apps, common, call of duty modern warfare 3, main
2. Remove the DLC folder, which is located in the ZONE folder.
3. Remove 42695.sdm file.
4. Remove these files: Localized_xxxx_iw06.iwd AND Localized_xxxx_iw07.iwd (NOTE: In the folder the "xxxx" should be a language, for example english, italian etc. If your game is in English, then remove Localized_english_iw06.iwd and Localized_english_iw07.iwd) To be sure, you could remove every localised_xxxx_iw06.iwd and localized_xxxx_iw07.iwd.
5. Remove both iw_25.iwd AND iw_26.iwd
This has helped me and I haven't got the error "Files are different from server" once after I did this (Needed to restart my game every 2nd to 3rd game before doing this)
This will, as I mentioned, remove your DLC
Hope this helped you.
I do NOT take any credits for this, as I did not find out this my self, but I hope this will help you guys who are still struggling.