vscode-docs/docs/getstarted/userinterface.md at 463630e70436308da191b917656571c7afb78797 · microsoft/vscode-docs · GitHub explains how this is accomplished in VS Code.
Relevantly, if/when 94978 – (Markdown) [Request] Save as Markdown and/or 160360 – Syntax Highlighting for Markdown is/are implemented, this shall become especially important to me, although I also export ODT files directly as HTML, so that would be too.
However, it obviously isn’t limited to languages which comparable editors support, nor even plain text files — I’d frequently like to be able to preview PDF export of an ODT file.