This scope lets an app "Upload, edit, and delete files as your slack app"
For new Slack apps, files:write replaces both files:write:user and files:write:bot. Here's a bit more detail on why perspectival scopes are disappearing.
This OAuth permission scope works with these Web API methods.
files.comments.delete - Deletes an existing comment on a file.files.delete - Deletes a file.files.revokePublicURL - Revokes public/external sharing access for a filefiles.sharedPublicURL - Enables a file for public/external sharing.files.upload - Uploads or creates a file.