Docs: clarify supported file types and directory handling in the desktop File pane

The desktop docs (code.claude.com/docs/en/desktop.md) describe two preview surfaces: the File pane (opens when a file path in chat/diff is clicked, for general file editing) and the Browser pane (explicitly documented to support static HTML, PDF, images, and videos).

Two things aren't covered by the docs and would help third-party integrations/agents built on top of Claude Code:

1. Which file types the File pane can render vs. fall back to plain text vs. error on (e.g. Markdown, JSON, CSV, Jupyter notebooks .ipynb, Office documents .docx/.pptx/.xlsx).
2. What happens when a directory path (not a file) is passed to the File pane. In testing, clicking a markdown link pointing at a directory produced "이 파일을 읽을 수 없습니다 / 삭제되거나 이동되었거나 작업 디렉터리 외부에 있을 수 있습니다" (cannot read this file / may have been deleted, moved, or is outside the working directory) instead of opening a directory listing — even though the same panel supports a tree/explorer view when opened directly via the Files tab.

Documenting the supported-type matrix and the directory-path behavior would help.