Renders the durable file attachments introduced in #24280 in the chat
interface. Without this, attachments were stored and served correctly
but the UI showed raw file parts with no previews or download UX.
Every attachment gets a download affordance, split into three rendering
tiers:
- **Images** — thumbnail with a hover/focus overlay containing a
download link. `onFocusCapture`/`onBlurCapture` with
`contains(relatedTarget)` keeps the overlay open while tabbing between
the image and its download link.
- **Text-like files** (`text/*`, `application/json`) — expandable
preview button with loading + error-with-retry states and the same
download overlay. Preview fetches throw a typed
`FetchTextAttachmentError` with a `.status` field instead of a
stringly-typed error.
- **Everything else** — compact `FileCard` with extension badge,
filename, and download link.
User-side and assistant-side rendering now share `AttachmentBlocks.tsx`
(`AttachmentPreviewFrame`, `TextAttachmentButton`,
`ImageAttachmentButton`, `FileCard`, plus
`getAttachmentHref`/`getAttachmentName`) instead of two near-duplicate
implementations. The text-attachment overlay anchors to the preview
surface so the download button stays pinned even when a loading/error
status line widens the row below.
`ComputerRenderer` detects when a screenshot was stored as a durable
attachment (`attachment_file_id`) and suppresses the stale base64
rendering — the screenshot appears as a proper file part instead.
`ToolLabel` shows the attached filename for `attach_file` tool calls.
Storybook coverage in `ConversationTimeline.stories.tsx` was expanded to
cover every tier (single/multiple images, inline + file-id text, JSON,
download-only files, fetch-failure retry, mixed attachments + file
references) with play-function assertions.
<img width="811" height="150" alt="image"
src="https://github.com/user-attachments/assets/27c71081-3502-4e80-92a7-d8adf1ff9323"
/>
## Cleanup
Per Mathias' post-merge suggestion on #24280, this PR also relocates
`coderd/chatfiles` → `coderd/x/chatfiles` so the durable-attachment
helpers live beside the rest of the `chatd` experimental surface.
Closes CODAGT-91
Agents can already see workspace files and take screenshots, but users could not download those artifacts from chat. This PR adds durable chat attachments to chatd. `attach_file`, explicit `computer` screenshot actions (not the automatic post-action screenshots), and `propose_plan` now fetch bytes over the agent connection, store them in `chat_files`, link them to the chat, and carry attachment metadata in tool responses so `buildAssistantPartsForPersist` can materialize ordinary `type:"file"` assistant parts that the chat file APIs serve.
The same storage helpers are reused for other artifact-producing paths. `wait_agent` recordings and thumbnails are stored as chat files and linked back to the parent chat, with best-effort relinking so parent chats retain those artifacts without leaving orphaned rows when chat-file caps reject links. `storeChatAttachment` wraps insert + link in one transaction, files are capped at 10 MB each and 20 per chat, and serving defaults to `Content-Disposition: attachment` with an explicit inline-safe allowlist.
This PR also consolidates chat-file media policy in `coderd/chatfiles`. Uploads and tool-generated attachments share byte-based MIME detection, SVG blocking, inline-safety rules, and compatible `text/plain` refinement for JSON, CSV, and Markdown. Prompt construction still only inlines synthetic pasted text for model consumption; assistant-created attachments are persisted for the user and intentionally not replayed into later LLM turns.
UI follow-up lives in #24281.
Relates to CODAGT-91
Ethan