I am using Linux Mint 17 Cinnamon 64 bit and recently got this error,
“Ignoring file ‘getdeb.list.bck’ in directory ‘/etc/apt/sources.list.d/’ as it has an invalid filename extension”
So fix this I used this simple command. It moves all the files ended with .bck in /etc/apt/sources.list.d directory to your ~/Desktop directory.
sudo find /etc/apt/sources.list.d/ -type f -name "*.bck" -exec mv {} ~/Desktop \;