Markdown (.md), plain text with syntax highlighting, shell scripts, .env files, JSON, YAML, TOML, archive contents (zip, tar, gz), and several more depending on version.

Peek works entirely as a Quick Look plugin — there is no app to open or configure. It activates automatically when you press Space on a supported file type.

If Quick Look stops working for a type, run qlmanage -r in Terminal to reset the Quick Look daemon and re-register all plugins.