This scope lets an app "Upload, edit, and delete files as your slack app"
For new Slack apps,
files:write replaces both
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 file
files.sharedPublicURL- Enables a file for public/external sharing.
files.upload- Uploads or creates a file.