The example that you give is not complete as it does not state which field/property you treat with the action.
The same could probably be achieved with an action of the type "Format value" for _EXTENSION and
Format string: $lower(%_extension%)
Or an action of the type "Case conversion" for the same field.
Provided, this would work at all.
I tried the action and I see no difference.
Thanks for the feedback, I have updated the field used above;
I have the latest version and my first thought was indeed to use case conversion but for some reason, I just don't see the _EXTENSION field as available. Probably missing something obvious in settings, will check again...
You are right, my Album NAMe (WITH CAPITAL AnD MixEd Letters) and UPPERCASE EXTENSION.MP3
becomes Album NAMe (WITH CAPITAL AnD MixEd Letters) and UPPERCASE EXTENSION.mp3
using "Replace with regular expression":
There is no additional fields or placeholders required for this action. Just place it inside the actions folder, which is located here: %AppData%\Mp3tag\data\actions or C:\Users%username%\AppData\Roaming\Mp3tag\data\actions