# POST /api/v2/work-items/{work_item}/attachments

Attach an uploaded file to a work item

- MCP id: work-items.attachments.store
- Scope: api:write:project_management
- Permission: project_management::manage_work_items
- Rate cost: 1 unit, write bucket. A write costs the same whatever `view` you ask for.
- Cache-Control: private, no-store

Step 3 of 3.

Turns bytes you have already uploaded into an attachment on the work item named in the path, and returns the file in the detail view. `upload_id` is the only key this accepts: create a ticket with `POST work-items/{work_item}/uploads`, PUT the raw bytes to its `upload_url`, then send its id here. Everything about the file (its name, its size, its media type, its collection and its storage location) is read off the ticket and off the bytes that actually arrived, so `file_name`, `mime_type`, `byte_size`, collection and disk are all a `422`. The ticket must have been created for this work item, by this API token, and must not have expired or been registered already; each of those is a specific `409` or `422` naming the cause. The server re-hashes the stored bytes and compares them with what it received before creating anything. This works exactly like an upload in plnnrly, so thumbnails and notifications follow identically. Attaching requires `project_management::manage_work_items`, the same permission plnnrly checks. `Idempotency-Key` is required: a lost 201 retried without one would report a failure for a create that succeeded. There is no `Location` header (a file has no item URL of its own), and no PATCH or DELETE on a file; remove one in plnnrly.

## Parameters
- work_item (integer or string, path): The WorkItem's numeric id. Or its issue_key, matching `[A-Z][A-Z0-9]*-[0-9]+`.
- view (string, optional): Which field set to render.
- X-Request-Id (header, optional): A correlation id of your own. It is recorded in our logs next to the server's request id but never returned: the response header always carries the server's id. A value that does not match the pattern is discarded silently, never rejected.
- Idempotency-Key (header, required): A caller-chosen key that makes this POST safe to retry. Required on this operation: omitting it is 400 idempotency-key-required.

## Request body
- upload_id (integer → Upload.id, required): The id of the upload ticket whose bytes become this file. Create one with `POST work-items/{work_item}/uploads` or `POST drive-items/{drive_item}/uploads`, PUT the bytes to its `upload_url`, then send its id here. Write-only: it is never rendered on a media row, and it is the only key either attachment store accepts.

Unknown keys are refused with `422` validation-failed, naming the key.

## Response 201
- id (integer, required): Unique identifier.
- name (string ≤ 255, required): File name as uploaded.
- mime_type (string or null ≤ 255, required): IANA media type.
- size (integer, required): Size in bytes.
- disk (string ≤ 255, required): Name of the storage location the file is kept in.
- collection (string ≤ 255, required): Media collection the file belongs to.
- created_at (timestamp (YYYY-MM-DDThh:mm:ssZ), required): Upload timestamp.
- updated_at (timestamp (YYYY-MM-DDThh:mm:ssZ), required): Last-write timestamp; use for incremental sync.
- url (string or null, optional, on request): Signed download URL, requiring your bearer token. Request with `?include=url`. Null only where your organisation's file storage cannot sign URLs, in which case the response carries a `signed_url_unavailable` warning and downloads are not available through the API.
- url_expires_at (timestamp (YYYY-MM-DDThh:mm:ssZ) or null, optional, on request): When `url` stops working. Always `null` in v2.0: the signed route the URL points at does not expire. Declared so a future bounded-TTL download path is an additive change.

## Errors
- 403 forbidden-by-policy | insufficient-permission | insufficient-token-scope: Forbidden.
- 409 idempotency-key-in-flight | idempotency-key-reused | state-conflict: Conflict.
- 415 unsupported-media-type: Unsupported media type.
- 422 unprocessable-relationship | validation-failed: Unprocessable content.
- 400 idempotency-key-required | invalid-cursor | invalid-query-parameter | malformed-request | unsupported-parameter: Bad request.
- 401 token-expired | unauthenticated: Unauthenticated.
- 404 endpoint-not-found | resource-not-found: Not found.
- 406 not-acceptable: Not acceptable.
- 429 rate-limit-exceeded: Too many requests.
- 500 internal-error: Server error.
- 503 tenant-unavailable: Service unavailable.

## Example
```bash
curl -X POST \
  'https://<your-url>/api/v2/work-items/1/attachments' \
  -H 'Authorization: Bearer <token>' \
  -H 'Accept: application/json' \
  -H 'Content-Type: application/json' \
  -H 'Idempotency-Key: <key>' \
  -d '{
    "upload_id": 1
  }'
```