How to lowercase file extensions automatically on Mac

What this does

Some uploaders and scanners hand back files named like “IMG_0042.JPG” or “Report.PDF”, an uppercase extension that looks inconsistent sitting next to everything else in a folder. This rule leaves the rest of the filename exactly as it is and only lowercases the extension, so “Photo.JPG” becomes “Photo.jpg”.

What you’ll build

A rule that matches every real file, then lowercases just its extension.

Open the rule editor

This walkthrough happens inside FilesAutomator itself, so install it from the Mac App Store first if you have not already, then open it. In its toolbar, choose Add, then New Automation Rule.

Name the rule and choose a folder

Name it something like “Lowercase my file extensions” and use Add Folder to choose the folder you want this applied to.

Build the condition

Add a single condition: Size is greater than 0, matching any real file.

FilesAutomator

Add the action steps

Step 1: Rename

Choose Add Step, then Rename. In “New name, built from the parts below”, use Add Part and choose Original name, then Add Part again, choose Text, and type a period. Add Part once more, choose Transform, then lowercase, and on that chip choose “the extension” as what it applies to.

FilesAutomator

Turn it on

Select Create Automation Rule, then switch the rule on. Every file that lands here now gets a consistently lowercase extension.

A faster way

The template gallery inside FilesAutomator already has this rule ready to install, named Lowercase file extensions.