> ## Documentation Index
> Fetch the complete documentation index at: https://www.mill.fyi/llms.txt
> Use this file to discover all available pages before exploring further.

# Complete an attachment upload

> Verify the uploaded image/PDF and attach it to the task. Invalid bytes, size or type are rejected; reuse the `Idempotency-Key` header on retries.

Write access: a member/administrator with a write-enabled credential, or an authorized signed-in user.

Verify the uploaded image/PDF and attach it to the task. Invalid bytes, size or type are rejected; reuse the `Idempotency-Key` header on retries.

Checks byte length, detected MIME/signature and optional SHA-256. Mill copies verified bytes to a final object. An expired upload returns 410 `UPLOAD_EXPIRED`; start a new upload.

MCP clients can use [`complete_attachment`](https://www.mill.fyi/mcp-reference/complete_attachment) for this operation.

[Request conventions](https://www.mill.fyi/api-reference/overview.md) · [API keys](https://www.mill.fyi/api-keys.md) · [MCP connections](https://www.mill.fyi/mcp-connections.md)


## OpenAPI

````yaml api-reference/openapi.json POST /api/tasks/{taskId}/attachments/{attachmentId}/complete
openapi: 3.1.0
info:
  title: Mill HTTP API
  version: 1.0.6
  description: >-
    All registered task-board, account, OAuth, MCP transport and health
    operations. MCP tool schemas come from runtime validators; account
    operations follow their server handlers. Example data is fictional.
    Generation imports routes without issuing requests or connecting to a
    database.
servers:
  - url: https://mill-api.example.com
security: []
paths:
  /api/tasks/{taskId}/attachments/{attachmentId}/complete:
    post:
      tags:
        - Attachments
      summary: Complete an attachment upload
      description: >-
        Verify the uploaded image/PDF and attach it to the task. Invalid bytes,
        size or type are rejected; reuse the `Idempotency-Key` header on
        retries.


        Checks byte length, detected MIME/signature and optional SHA-256. Mill
        copies verified bytes to a final object. An expired upload returns 410
        `UPLOAD_EXPIRED`; start a new upload.


        Write access: a member/administrator with a write-enabled credential, or
        an authorized signed-in user.
      operationId: post_api_tasks_by_taskId_attachments_by_attachmentId_complete
      parameters:
        - name: taskId
          in: path
          required: true
          schema:
            type: string
            format: uuid
          description: >-
            The taskId from the corresponding list, record or authorization
            response.
        - name: attachmentId
          in: path
          required: true
          schema:
            type: string
            format: uuid
          description: >-
            The attachmentId from the corresponding list, record or
            authorization response.
        - name: Origin
          in: header
          required: false
          schema:
            type: string
            format: uri
          description: >-
            The exact configured Mill UI origin. Required for browser mutations
            carrying session cookies.
        - name: Idempotency-Key
          in: header
          required: false
          schema:
            type: string
            minLength: 8
            maxLength: 128
            pattern: ^[A-Za-z0-9._:-]+$
          description: >-
            Reuse the same key and exact request bytes for retries of one
            logical mutation; different content conflicts.
      responses:
        '200':
          description: >-
            Successful response. Examples are illustrative; field values vary by
            record.
          content:
            application/json:
              schema:
                type: object
                properties:
                  attachment:
                    type: object
                    properties:
                      id:
                        type: string
                      taskId:
                        type: string
                      fileName:
                        type: string
                      contentType:
                        type: string
                      sizeBytes:
                        type: integer
                      createdBy:
                        type: string
                      createdAt:
                        type: string
                    additionalProperties: true
                additionalProperties: true
                description: >-
                  Illustrative response shape. Fields can be absent in alternate
                  successful flows; see the operation description and example.
              example:
                attachment:
                  id: 44444444-4444-4444-8444-444444444444
                  taskId: 22222222-2222-4222-8222-222222222222
                  fileName: restore-report.pdf
                  contentType: application/pdf
                  sizeBytes: 2048
                  createdBy: 66666666-6666-4666-8666-666666666666
                  createdAt: '2027-01-18T09:30:00.000Z'
        default:
          description: >-
            Request failure. Inspect code, message and requestId; respect
            Retry-After on HTTP 429.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Error'
      security:
        - bearerAuth: []
        - sessionCookie: []
components:
  schemas:
    Error:
      type: object
      required:
        - error
      properties:
        error:
          type: object
          required:
            - code
            - message
            - requestId
          properties:
            code:
              type: string
            message:
              type: string
            requestId:
              type: string
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      description: Personal or team Mill REST API key
    sessionCookie:
      type: apiKey
      in: cookie
      name: mill_session
      description: >-
        Signed-in human browser session; mutation requests also need the exact
        UI Origin

````

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