fix: restrict PDF preview to PDF content type

[overtrue]

Pull Request

Description

Restricts the PDF viewer to objects served as application/pdf so extension-only .pdf matches fall back to the sandboxed preview path.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvements
• Security fix

Testing

• Unit tests added/updated
• Manual testing completed

CI=true pnpm install --frozen-lockfile
node --test tests/lib/object-preview-source.test.js
pnpm lint
pnpm type-check
pnpm format:check

Checklist

• Code follows the project's style guidelines
• Self-review completed
• TypeScript types are properly defined
• All commit messages are in English (Conventional Commits)
• All existing tests pass
• No new dependencies added, or they are justified

Related Issues

None

Screenshots (if applicable)

None

Additional Notes

None