I think the wallet-xxxxx.key file is from multibit classic and there is a tool to decrypt it if you want to export all private keys from that file and import it to other wallet like Electrum.
Try this tool
https://github.com/HardCorePawn/multibit_recoveryIt requires Python 2 so download it install
Now the downloaded multibit_recovery put them to the same folder and open a command prompt at the same folder(hold shift+right-click then select command prompt.).
Now use this command
python decrypt_multibit_classic_keys.py multibit-\<DATETIMESTAMP>.key
The result should be your extracted private keys.