How to do it
- Add the filesDrag them onto Batch File Renamer, or use the Finder Quick Action.
- Pick the "Fix .JPG to .jpg" recipeOr add an Extension step: lowercase, uppercase, set, add or remove.
- RenameNo per-file warning dialogs. Undo puts them back.
Fix extensions from the real file type
A file called photo.png that's really a JPEG confuses some apps. The Extension step can set each file's extension from its actual contents.
In Terminal
for f in *.JPG; do mv "$f" "${f%.JPG}.jpg"; done works for a single folder, with no preview and no undo.
Questions
How do I change many file extensions at once on Mac?
Use an Extension step in Batch File Renamer Lite (free), or a shell loop in Terminal.
Is it free?
Yes. Extension steps are in Lite, with no file limit.