fix: improve file preview fallback handling

[edgarsskore]

Summary

• Keep file preview loading after refresh instead of falling back to an unavailable message.
• Include returned file content in read_file structuredContent for preview consumers.
• Render unsupported file payloads as escaped raw content when structured content is available.

Tests

• npm run build
• node test/test-markdown-preview.js
• node test/test-file-handlers.js

Summary by CodeRabbit

Release Notes

• New Features

  • File previews now display raw content for unsupported file types when available, instead of showing an unavailability message.

• Bug Fixes

  • Improved preview reliability by removing artificial timeouts that previously marked previews as unavailable.
  • Enhanced text extraction from structured file previews across all file formats.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f3297522-914b-4191-ba2a-ca8e4cb0e018

📥 Commits

Reviewing files that changed from the base of the PR and between 5c30886 and ddbe8fc.

📒 Files selected for processing (7)

• src/handlers/filesystem-handlers.ts
• src/types.ts
• src/ui/file-preview/src/app.ts
• src/ui/file-preview/src/file-type-handlers.ts
• src/ui/file-preview/src/payload-utils.ts
• test/test-file-handlers.js
• test/test-markdown-preview.js

💤 Files with no reviewable changes (1)

• src/ui/file-preview/src/app.ts

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📝 Walkthrough

Walkthrough

The PR integrates structured content field population across file handlers and UI layers. A new optional content: string field is added to the FilePreviewStructuredContent interface, populated differently for PDFs (newline-joined text), images (image data), and other files (text content). UI extraction and rendering logic now prefer this structured field over legacy extraction methods. Unsupported file handling becomes content-aware, enabling previews when raw content is available. App initialization is simplified by removing a timeout-based fallback.

Changes

File preview structured content integration

Layer / File(s)	Summary
Data contract and handler implementations src/types.ts, src/handlers/filesystem-handlers.ts, test/test-file-handlers.js	Added optional content?: string field to FilePreviewStructuredContent. Handlers populate it with PDF text (text entries joined by newlines), image data, or plain text. Tests validate the contract is fulfilled for all file types.
UI payload extraction and structured-content-first fallback src/ui/file-preview/src/payload-utils.ts	New extractStructuredContentText helper extracts the content field from structured payloads. extractRenderPayload now prefers structured content over tool-extracted text in its fallback chain.
Unsupported file content-aware behavior and preview test src/ui/file-preview/src/file-type-handlers.ts, test/test-markdown-preview.js	Unsupported handler determines preview and copy capabilities based on whether stripped content is non-empty; renders empty placeholder or escaped-content display. New test covers payload extraction, capability detection, and rendered output validation.
App initialization and host context setup src/ui/file-preview/src/app.ts	Removed 8-second timeout fallback from host-connection handling. Directly initializes currentHostContext and seeds persisted payload state without intermediate unavailability markers.

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• wonderwhy-er/DesktopCommanderMCP#339: Both PRs update FilePreviewStructuredContent handling for images, moving imageData usage into structuredContent with related UI/payload logic.
• wonderwhy-er/DesktopCommanderMCP#337: Both PRs modify handleReadFile and extend the FilePreviewStructuredContent contract for structured payload integration.

Suggested labels

size:L

Suggested reviewers

• wonderwhy-er
• scutuatua-crypto

Poem

🐰 A content field takes its place,
through handlers, PDFs, images with grace.
UI layers extract with care,
unsupported files now shine fair.
State flows pure, no timeout despair! 📄✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title 'fix: improve file preview fallback handling' directly summarizes the main objective: improving how file preview handles fallback scenarios (preventing unavailable messages on refresh and rendering unsupported content).
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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• Commit unit tests in branch fix/ui-ux-improvements

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