Avast released a decryptor for CryptoMix victim's that were encrypted while in offline mode. Offline mode is when the ransomware runs and encrypts a victim's computer while there is no Internet connection or the computer cannot connect to the ransomware's Command & Control server.
If your computer was infected by CryptoMix when you were not connected to the Internet, then you may be able to decrypt your files for free using this decryptor. A guide on how to use this decryptor can be found here: Avast Releases a Decryptor for Offline Versions of the CryptoMix Ransomware.
This decryptor currently supports the decrypting of offline CryptoMix infections that append the following extensions to encrypted files:
.CRYPTOSHIELD .scl .rscl .lesli .rdmk .code .rmd