Add File System Access API (Platform-Backed Subset)

[FlorianRappl]

Proposal

Provide a practical File System Access subset for file open/save and writable handles using injected platform services.

Scope:

• Add showOpenFilePicker and showSaveFilePicker.
• Add FileSystemFileHandle and writable stream subset.
• Support read, write, and close operations.
  Technical Notes:
• All filesystem operations must go through a platform interface.
• Keep secure-context and permission checks configurable.
  Acceptance Criteria:
• Picker calls return handles with stable identity semantics.
• Write workflows are transactional enough for tests.
• Unit tests cover open, save, overwrite, and permission-denied paths.
  Out of Scope:
• Full directory picker and advanced handle graph in first pass.
• OS-native picker UX fidelity.

Description

Add a platform-backed File System Access subset: showOpenFilePicker, showSaveFilePicker, and file-handle write flows with explicit permission checks.

Background

This feature enables realistic file import/export automation and desktop-like workflows in browser-driven apps while preserving host control via injected platform services.

Specification

• File System Access API draft: https://wicg.github.io/file-system-access/
• Initial scope: file picker + writable file handle subset