# Document Preview

## Objective

Show a readable preview of an uploaded file without opening another program.

Server-generated previews for common uploaded document types, shown inline wherever files are listed.

## 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. Decide the small set of types worth previewing based on what users actually upload, and treat everything else as an icon plus file details. A general-purpose converter for every format is not the minimum coherent version of this.
2. Generate previews in the app's existing background job system after upload completes, and show the file as uploaded with a preview pending rather than holding the upload open.
3. Extend the app's existing File Upload and Attachment Gallery to display previews in place, rather than adding a second file surface alongside them.
4. Authorise the preview exactly as strictly as the source file. A generated thumbnail of a private contract is the private contract.
5. Do not render documents by handing the original bytes to the browser. Convert server-side and serve the rendered output, so an uploaded file cannot execute anything in a viewer's session.

## 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

- Conversion is slow and sometimes fails. Queue it in the background, show progress and a clear failed state, and never let a conversion block or roll back the upload itself.
- Very large files, files with hundreds of pages, and archives nested many levels deep will exhaust a worker. Cap page count, file size, and nesting depth, and say the preview was truncated rather than failing silently.
- Cache generated previews and expire them when the source file is replaced, or users will keep seeing the old version of a document they have already updated.
- Unsupported types must degrade to a file type icon with name, size, and date, which is genuinely useful, rather than a broken image frame.
- Documents embed scripts, macros, and remote references that phone home when rendered. Strip them during conversion and block outbound fetches from the renderer.
- Previews are derived files that also consume storage. Give them a retention policy and delete them when the source file is deleted, including from any cache or CDN.
- If the app already runs Upload Malware Scanning, previews must not be generated or shown until the scan clears, otherwise the preview becomes the delivery mechanism.
- Reserve the preview's dimensions in the list so a page of files does not reflow as each thumbnail arrives, and give every preview alt text naming the file.

## 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

- [ ] Common uploaded types show a readable inline preview where files are listed.
- [ ] Conversion runs in the background with visible progress and never blocks upload.
- [ ] Oversized and deeply nested files are capped and reported as truncated.
- [ ] Previews are cached and regenerated when the source file changes.
- [ ] Unsupported types fall back to an icon with file details.
- [ ] Embedded scripts and remote references are stripped before rendering.
- [ ] Deleting a file removes its previews, and preview access requires the same permission as the file.
- [ ] 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.