File Explorer Add ons Overview
File Explorer add-ons, right now are just limited to Preview Pane additions for File Explorer.
Preview Pane is an existing feature in the File Explorer which shows a lightweight, rich, read-only preview of the file's contents in the view's reading pane.
To enable it, you just click the View tab in the ribbon and then click Preview Pane.
Below is an example of Markdown and Svg files previews in File Explorer with PowerToys.
