If I go to the archived version [0] of the site and click the + button I end up at [1]
[0] https://web.archive.org/web/20210329151816/http://url-wiki.c...
[1] https://web.archive.org/web/20210329151816mp_/http://url-wik...
Edit: on closer inspection you can overwrite the whole index page just by passing in URL-encoded HTML in one of the form fields.
Edit 2: It's even worse than I thought. Arbitrary file upload. Flagging this post for its own good.