Not a problem if the page already exists, e.g. https://en.wikipedia.org/wiki/97.7_FM?veaction=edit
This seems related to T232556: Parsoid/PHP roundtrip testing: Period ( . ) in titles fail with a http 403 (Invalid file extension found in the path info or query string.) error. But now that IE6/IE7 support has been removed, it is possible this will unbreak once that code rides the train.