They did in fact file rape charges, but as is common they didn't do it immediately but after talking about the event with friends.

They have also, wisely avoiding much of the media, enlisted a lawyer to speak on their behalf on the matter, and it was the lawyers filing that led to the investigation being reopened again.

For a pretty complete chronology, see this page on the Swedish prosecution authority: https://www.aklagare.se/en/news-and-press/media/the-assange-...