How to fix N: Ignoring file 'filename' in directory '/etc/apt/sources.list.d/' as it has an invalid file name extension in APT

First, identify which files in /etc/apt/sources.list.d/ have invalid extensions:

ls -la /etc/apt/sources.list.d/

Look for files that don't end with .list or .sources. Files with extensions like .bak, .save, .disabled, .txt, or backup suffixes (like ~) are the problem files.

Delete files that are backups or temporary files:

sudo rm /etc/apt/sources.list.d/*.bak
sudo rm /etc/apt/sources.list.d/*.save
sudo rm /etc/apt/sources.list.d/*~
sudo rm /etc/apt/sources.list.d/*.swp

If you have specific invalid files, remove them individually:

sudo rm /etc/apt/sources.list.d/invalid-filename.txt

If you intentionally disabled a source by renaming it (e.g., to .disabled), you have two options:

Option A: If you want to keep the source but have it ignored, rename it to have a .list extension:

sudo mv /etc/apt/sources.list.d/google-chrome.disabled /etc/apt/sources.list.d/google-chrome.list

Option B: If you want to permanently disable the source, prefix the filename with a dot (hidden file):

sudo mv /etc/apt/sources.list.d/google-chrome.list /etc/apt/sources.list.d/.google-chrome.list

After renaming or removing files, run apt update to confirm the warning is gone:

sudo apt update

The 'Ignoring file' notices should no longer appear. If you see the message again, double-check that all files in /etc/apt/sources.list.d/ end with .list or .sources.