It is possible to add custom metadata to media. Am I right that this works only for media, but not for files? Is there a way to add additional metadata to files. It would be nice to use a custom name in a file list instead of the original filename https://twillcms.com/docs/media-library/custom-metadata.html

You're correct, it only works for medias fields (images in the media library) at the moment. It would make sense though. For now the approach would be to use another field next to your file field.