[BUG] Images sent by agent via SendUserFile never render in Desktop app chat (local session, claude.ai webview)

Environment

• Claude Desktop for Windows (MS Store build) 1.12603.1.0, bundled Claude Code 2.1.170
• Windows 11 Pro 10.0.26200
• Local session (working dir on local disk), chat UI = claude.ai webview inside the desktop app

What happens

When the agent sends image files to the user with the SendUserFile tool, the tool reports success ("N files delivered to user") and the session transcript records correct attachment metadata. But nothing renders in the chat UI — no inline image, no usable preview.

Tested with PNGs from 22 KB (1280x413) up to 6 MB (10000px tall) — none render. So this is not the API image limits (5 MB / 2000px): fully compliant small images fail the same way.

Evidence

Transcript record (session .jsonl) for one of the calls:

{"caption":"...","attachments":[{"path":"F:\\...\\starter-1.png","size":22772,"isImage":true,"media_type":"image/png"}]}

• %APPDATA%\Claude\logs\main.log contains no errors and no file-load attempts around delivery time — silent failure.
• Renderer log (claude.ai-web.log) has no entries at delivery time either.

Likely root cause

Same architecture gap as #48466: the chat UI talks to the local session through the API bridge, which only streams messages and tool results (text). There is no channel for binary file content, so the claude.ai-hosted UI can never fetch local-path attachments. #48466 covered the preview pane in remote-control sessions and was closed as not planned — but this affects inline rendering of agent-sent images even in a plain local desktop session, where SendUserFile is the primary way for an agent to show generated artifacts (screenshots, diagrams, comparisons).

Expected

Images delivered via SendUserFile render inline in chat, or at minimum appear as an attachment chip whose preview opens.

Repro

1. Desktop app, local session.
2. Ask the agent to create a small PNG and send it with SendUserFile.
3. Tool result says "1 file delivered to user" — chat shows nothing.

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Investigated and drafted by Claude Code (transcript + local logs on the affected machine), filed by the user.