Possibly unintended support for other than `md` extensions?

[arpadgabor]

I was looking through the repo and saw this line that I thought would mean this widget only works for md files. Tested it with json files and it seems to work.

netlify-cms-widget-parent/src/ParentWidget.js

Line 21 in 788722c

return `${collectionFolder}/${value}/${indexFile}.md`;

Not sure if there is something to be fixed here, but I wanted to bring this up in case this could cause problems down the road.