I have the same problem. I uploaded a file that was 25mb and had a + in the filename. I’m not sure if it broke because there were more than 5 million tokens or filename issues.
Cannot delete through web UI.
You can try deleting by the API itself. First list files, obtaining file names to match up with the returned file IDs, then send a delete API call for the ID.
Here’s a basic Python utility for doing that. It is hard-coded for purpose:assistants. And doesn’t employ pretty pagination if you have 10000 files to list out.