WordPress permalinks – making them work with /category-name/index

Hi

I’ve recently started migrating some sites from Movable Type to WordPress, and have recently found an interesting issue.

With my old site, the permalinks for categories were officially of the format
https://somesite.com/category-name/index

Now generally they were presented to the user as
https://somesite.com/category-name/

However it’s possible that some URLs did go out in the old format.

The problem I have is that now in WordPress (set up with Multisite)
https://somesite.com/category-name/index
gives a 404.

But https://somesite.com/category-name/index.php does

I would like to find a way to support the old style URLs so that they just work without the .php being there. Does anyone know? I’ve tried a search for this however it’s very difficult to find anything relevant.

Many thanks