feat: support markdown in file tree notebook creation

[daizutabi]

📝 Summary

This is a follow-up PR to #8412 ("Add Create notebook in file tree").

While #8412 introduced the ability to create new marimo notebooks directly from the file explorer, it defaults to Python boilerplate even when the user specifies a .md or .qmd extension.

This PR enhances the "Create notebook" functionality to:

• Detect the file extension during creation.
• Automatically generate the appropriate boilerplate using MarimoConvert if the extension is .md or .qmd.

🔍 Description of Changes

• Modified marimo/_server/files/os_file_system.py: Update create_file_or_directory to handle Markdown-based notebook initialization.
• Updated tests/_server/files/test_os_file_system.py: Added test cases to verify the generated boilerplate content for .py, .md, and .qmd files.

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• Tests have been added for the changes made.
• Documentation has been updated where applicable, including docstrings for API changes.
• Pull request title is a good summary of the changes - it will be used in the release notes.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Mar 21, 2026 6:23am

[Copilot]

Pull request overview

Enhances server-side “Create notebook” behavior in the file explorer so that new notebooks created with .md / .qmd extensions are initialized with Markdown/Quarto boilerplate instead of Python boilerplate, aligning backend creation with supported notebook formats.

Changes:

• Updated notebook creation to generate .md/.qmd boilerplate via MarimoConvert (from IR) and keep .py generation via Python conversion.
• Added parametrized tests to validate the created boilerplate differs across .py, .md, and .qmd.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
marimo/_server/files/os_file_system.py	Detects extension on notebook creation and uses MarimoConvert to emit Markdown/Quarto vs Python boilerplate.
tests/_server/files/test_os_file_system.py	Adds parametrized coverage to confirm correct boilerplate markers for .py, .md, .qmd notebook creation.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.