I updated to nightly build (5/29 appears to be the latest:
https://build.koreader.rocks/download/nightly/) to get the "filtering" feature in collections. On the stable release, files other than epub and pdf were being added to the collection with the ""Add all books from a folder and its subfolders" function (for example all the jpg cover images). Thank you huis07 for that commit (
https://github.com/koreader/koreader/pull/13724)!