Desktop app: SendUserFile markdown cards show download dialog instead of opening in MD viewer

Summary

In the Claude Code desktop app (macOS), clicking a markdown file card sent via SendUserFile now shows a download dialog instead of opening the file in the built-in markdown viewer. This used to work (the file opened inline in the side-panel MD viewer) until around late July 2026.

Environment

• Claude Code desktop app (macOS): 1.32885.1 (no app update available at time of report)
• Session runtime CLI @anthropic-ai/claude-code: reproduced on 2.1.195 and, after claude update, on 2.1.237 (latest at time of report) — so the CLI version does not appear to be the cause
• OS: macOS (Darwin 25.2.0)

Steps to reproduce

1. In a desktop-app session, have Claude send a small markdown file with the SendUserFile tool (display: "render" explicitly set).
2. Click the resulting file card in the conversation.

Expected

The markdown opens in the app's built-in MD viewer panel (previous behavior, confirmed working ~2026-07-21).

Actual

A download dialog appears; the file is never rendered in the viewer.

Notes

• Reproduces with a tiny 2 KB markdown file, so it is not size-related (first noticed with a 1.1 MB file, then isolated with the small one).
• display: "render" is explicitly passed and does not change the behavior.
• CHANGELOG for v2.1.222 mentions changes to file-delivery card upload handling (Remote Control context) — possibly related, but reproduction on 2.1.195 predates installing that version locally.