ACTIONS that mimics a WYSIWYG to Change File Extension or File Name and Extension (2022-2023)

General Discussion
4

I also do not see the problem. We can change the filename and/or extension with a regular expression, as already mentioned here.

To illustrate it:
Extension and Filename to lowercase

Action "Replace with regular expression" to change only the extension to lowercase:
Field : _FILENAME
Regular expression: \.(.*)$
Replace matches with: .$lower($1)

Action "Replace with regular expression" to change the filename including the extension to lowercase:
Field : _FILENAME
Regular expression: (.*)
Replace matches with: $lower($1)

Update 22.02.2023, 08.45:
As @wneclass2018 stated later in this thread, there is a problem with the above regular expressions, if the filenames include (multiple) dots. Please use the updated versions in this answer.