This action file, below, will solve your problems, and if you're looking to change the file extension of a file, in a WYSWYG manner, visit this community post: FINALLY SOLVED: ACTIONS that mimics a WYSIWYG to Change File Extension or File Name and Extension (2022-2023)
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
CONVERT File Extension (%_extension%) to Lowercase by wneclass2018 (February 20th 2023).mta (759 Bytes)