The Project Bin file picker, when clicking "Add Clip or Folder", has a filter for "All Supported Files". However, I found that this filter does not include .webp, which, after disabling that filter and adding a .webp file to my project, works fine in Kdenlive. That filter should therefore include .webp files.
A possibly relevant merge request was started @ https://invent.kde.org/multimedia/kdenlive/-/merge_requests/223
Thanks for your contribution! Fixed in master now!