# Attachment Gallery

## Objective

Make attached files browsable instead of a list of filenames.

A gallery surface for the files attached to a record, with per-type previews, stable ordering, captions, and per-item permissions.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Render every attachment as a tile with a type-appropriate preview: a thumbnail for images, a first-page or icon preview for documents, a duration badge for media.
2. Define a fallback for every type you do not preview, showing filename, size, and type. An unrecognised file must still be identifiable.
3. Only preview formats you have decided are safe to render inline. Do not inline arbitrary HTML or SVG from user uploads — it executes in your origin.
4. Give attachments an explicit order value rather than relying on creation time, and let the user reorder and caption them.
5. Check download permission per item at request time, not once for the whole record.
6. Do NOT build an upload control here — that is owned by File Upload. The fullscreen image viewer is owned by Image Lightbox, and authorized delivery of the file bytes is owned by Secure File Downloads. Extend those rather than duplicating them.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- An attachment still processing must show a processing tile, not a broken thumbnail.
- A file deleted while the gallery is open must fail cleanly on click, not 500.
- A quarantined or scan-failed file must be visible as blocked and must not be previewable or downloadable.
- Two people reordering at once must not silently overwrite each other — resolve on the server and refresh the client's order.
- A caption is user text rendered next to a file; escape it.
- A gallery with two hundred attachments must paginate or lazy-load rather than requesting two hundred thumbnails at once.
- Thumbnail generation failure must degrade to the fallback tile, not an empty box.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every attachment renders either a real preview or a labelled fallback.
- [ ] Processing, deleted, failed, and quarantined states each have their own visible treatment.
- [ ] Download permission is enforced per item, not per record.
- [ ] Order and captions survive concurrent edits without silent loss.
- [ ] No user-supplied file is rendered inline in a way that can execute script.
- [ ] Upload, fullscreen viewing, and file delivery are deferred to their owning features.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.