xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxxxxxxxxxxxxx"
Encrypted .key files are either generated by the user when they explicitly export their private keys from Multibit using the "Tools -> Export Private keys" option and set a password on the export file in the last section of the export window:
or
they are created automatically by Multitbit when you set/change the wallet password (it creates an automatic .key backup encrypted with the wallet password).
My advice would be to install MultiBit and try and open the .wallet file... hopefully there is no password set on the .wallet file and you'll be able to create a new "key export" but using the "Do not password protect export file" option... you'll likely have to wait for MultiBit to finish syncing, which could take quite a while (upwards of an hour)... but once you do that, you should end up with a "plaintext" .key file that you can open in a text editor and it will look something like this:
# Anyone who can read this file can spend your bitcoin.
#
# Format:
#
#
# The Base58 encoded private keys are the same format as
# produced by the Satoshi client/ sipa dumpprivkey utility.
#
# Key createdAt is in UTC format as specified by ISO 8601
# e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory
#
L4eBXv6aaE75ysQNMGfdWVWsoSikz8M1FcxeZa3Jx3P1Gyxy2MuX 2019-04-21T13:35:56Z
L52mJcSfHEA8qvYM2hGBR834zL65QD7Q6YYHviDUXSi989c7wBqJ 2019-04-21T13:35:56Z
# End of private keys