How to filter out the file extensions in lower case?

The macOS operating system is installed as a case-insensitive filesystem. That means that only one filename.ext may exist in a given namespace. You can have either cat.txt or cat.TXT, but not both in the same folder location. If you attempt to save cat.txt to cat.TXT, you will get the following warning from the Finder:

If I am in the Terminal and type the following where only cat.TXT exists:

ls cat.txt

the result will be shown in all lowercase. If I had typed that same string with the upper-case extension, that would have been shown.

The Finder can only show the exact filename case and does not offer you the ability to specify upper or lower-case extension views.

Ivon wrote:

I have file extensions in both uppercase and lowercase letters.

How to filter out the file extensions in lower case?

Different apps, different interactions can generate extensions differently....

Located where...?

Why try to and micro-manage file extensions?

Did you manage to format your drive to be case-sensitive(?) This I would not recommend, unless there is some compelling reason—otherwise you create your own headaches.

File system formats available in Disk Utility on Mac - Apple ...

Show or hide filename extensions on Mac - Apple Support

Can you provide a Screen Shot to better illustrate this behaviour / issue ?