If you want to change the case of the extension, try the following:
Open the "Tag" window in the lower left corner of MC.
If the "Filename (name)" entry is not visible, then, in the tag window, right click > also show > Filename (name)"
Now that you see it, select one file then click in the "Filename (name)" field in the tag window to open it for editing.
Paste the following in there: =clean([filename (name)])
Press enter.
Is that what you want?
If the answer is yes, feel free to select as many files as you like and repeat the steps above. Note, with multiple files selected, the field in the tag window will display [Varies]. It's fine to open that for editing and follow the steps.
If the answer is no, press ctrl+z to undo whatever just happened, then post back explaining what you did, what happened, and why you're not happy. With a wing and a prayer, we should be able to get you sorted.
-marko