Desktop app: broken thumbnail preview for image files sent via SendUserFile

Environment: Claude Code desktop app on Windows 11 (10.0.26200), July 2026 session (model claude-fable-5).

Bug: When the agent sends an image to the chat with the SendUserFile tool (PNG screenshots, ~300-900 KB, e.g. 2388x1153), the chat renders a file card with no thumbnail preview - just the generic "PNG" card with filename and size. Clicking the card opens the image correctly, so the file itself is intact; only the inline thumbnail preview is broken.

Expected: The file card should show an image thumbnail preview for common image types.

Steps to reproduce:

1. Agent calls SendUserFile with a .png path (no display parameter set).
2. The card appears in chat without a thumbnail.
3. Clicking the card opens the full image fine.

Reported on behalf of a user via Claude Code.

<img width="465" height="306" alt="Image" src="https://github.com/user-attachments/assets/9ea83032-d2aa-499a-bb40-d707285f2d6f" />