Here’s a very simple utility that recursively finds uppercase file extensions and makes those who are lowercase. For once, it’s not written in C# but in good old DOS. If you paste this code in a CMD file, you’ll have a working program. If you execute this cmd, it will take the current folder as the root and it will process all files and subfolders.
Nowadays you would use PowerShell but this is a way to provide a quick fix to your users.