media-preview: attempt to handle untitled scheme for Safari/Firefox (fix #236152)

[jhhtaylor]

Summary

This PR attempts to fix audio/video preview for files opened via Safari's "Open Files..." dialog on vscode.dev.

• Add handling for untitled: scheme in audioPreview.ts and videoPreview.ts
• Attempt to read file contents and create data URLs for playback
• Update CSP to allow data: URIs for media-src

Limitation

This fix cannot fully resolve the issue because workspace.fs.readFile() cannot access untitled: scheme files.

When files are opened via Safari's limited File API (browsers without full File System Access API support):

1. Contents are read via FileReader into memory
2. A URI with untitled: scheme is created
3. Custom editors only receive the URI, not the contents
4. The file system cannot resolve untitled: URIs

Root Cause (needs core fix)

The untitled: scheme is designed for text files, not binary files. A proper fix would require changes in VS Code's core - either:

• In how Safari's "Open Files..." stores binary file contents (e.g., use blob URLs or temporary files instead of untitled: scheme)
• Or how custom editors receive untitled document contents

Related Issues

• vscode.dev has bad behavior when opening an unknown file type #148353 - Same root cause affecting other binary files on Safari/Firefox

Test Plan

1. Open vscode.dev in Safari
2. Use File → Open Files... to open an audio/video file
3. Observe that the media preview attempts to load (shows player UI) but playback doesn't work due to the limitation above
4. Compare with Chrome where the full File System Access API is supported

[jhhtaylor]

@microsoft-github-policy-service agree