> ## Documentation Index > Fetch the complete documentation index at: https://sendmux.ai/docs/llms.txt > Use this file to discover all available pages before exploring further. # Create a presigned mailbox attachment upload > Creates a short-lived signed PUT URL for one attachment. The caller must be authenticated to mint the URL; the later PUT uses the signed URL, exact Content-Type, and exact Content-Length without sending an API key. The PUT returns a blob ID that can be supplied to `POST /mailbox/messages/send`. ## OpenAPI ````yaml /openapi-app.json post /mailbox/attachment-uploads openapi: 3.1.0 info: description: Programmatic access to your Sendmux email infrastructure. title: Sendmux API version: 1.0.0 servers: - description: API v1 url: https://app.sendmux.ai/api/v1 security: - bearerAuth: [] paths: /mailbox/attachment-uploads: post: tags: - Mailbox API summary: Create a presigned mailbox attachment upload description: >- Creates a short-lived signed PUT URL for one attachment. The caller must be authenticated to mint the URL; the later PUT uses the signed URL, exact Content-Type, and exact Content-Length without sending an API key. The PUT returns a blob ID that can be supplied to `POST /mailbox/messages/send`. operationId: mailboxCreateAttachmentUpload parameters: - description: >- Mailbox public ID to target when the credential grants access to more than one mailbox. Omit when the credential is scoped to exactly one mailbox. in: query name: mailbox_id required: false schema: type: string requestBody: content: application/json: schema: $ref: '#/components/schemas/MailboxAttachmentUploadIntentBody' responses: '200': content: application/json: schema: $ref: >- #/components/schemas/MailboxAttachmentUploadIntentResultResponse description: Attachment upload URL created '400': content: application/json: schema: $ref: '#/components/schemas/ApiError' description: Invalid upload request '413': content: application/json: schema: $ref: '#/components/schemas/ApiError' description: Attachment too large '503': content: application/json: schema: $ref: '#/components/schemas/ApiError' description: Attachment uploads unavailable security: - bearerAuth: [] components: schemas: MailboxAttachmentUploadIntentBody: additionalProperties: false properties: content_type: description: Content-Type that must be sent on the later PUT request. example: application/pdf type: string filename: description: Filename to use when sending the uploaded attachment. type: string size_bytes: description: Exact byte length that must be sent on the later PUT request. minimum: 1 type: integer required: - filename - content_type - size_bytes type: object MailboxAttachmentUploadIntentResultResponse: allOf: - $ref: '#/components/schemas/SuccessEnvelope' - properties: data: $ref: '#/components/schemas/MailboxAttachmentUploadIntentResult' meta: $ref: '#/components/schemas/ResponseMeta' required: - data type: object unevaluatedProperties: false ApiError: additionalProperties: false properties: error: additionalProperties: false properties: code: description: Machine-readable error code enum: - invalid_parameter - missing_parameter - authentication_required - insufficient_permissions - not_found - conflict - limit_exceeded - idempotency_conflict - payload_too_large - validation_error - rate_limit_exceeded - service_unavailable - internal_error example: invalid_parameter type: string doc_url: description: Link to relevant documentation type: string errors: description: >- Accumulated per-field issues for validation errors. Only present on 400/422 responses. items: $ref: '#/components/schemas/ApiErrorDetail' type: array message: description: Human-readable error description type: string param: description: The parameter that caused the error type: string retryable: description: >- Whether the caller may safely retry this request. 4xx are typically false (except 429); 5xx are typically true. example: false type: boolean required: - code - message - retryable type: object meta: additionalProperties: false properties: request_id: example: req_clxxxxxxxxxxxxxxxxxxxxxxxxx type: string required: - request_id type: object ok: enum: - false type: boolean required: - ok - error - meta type: object SuccessEnvelope: properties: meta: additionalProperties: {} properties: {} type: object ok: enum: - true type: boolean required: - ok - meta type: object MailboxAttachmentUploadIntentResult: additionalProperties: false properties: expires_at: description: Upload URL expiry time. format: date-time type: string headers: additionalProperties: false description: Headers that must be sent exactly with the PUT request. properties: Content-Length: type: string Content-Type: type: string required: - Content-Type - Content-Length type: object max_size_bytes: description: Maximum accepted attachment size in bytes. type: integer method: description: HTTP method to use with `upload_url`. enum: - PUT type: string upload_id: description: Short-lived upload intent ID. type: string upload_url: description: >- Short-lived signed PUT URL for this exact attachment. Upload promptly; if it expires, create a new upload URL. example: >- https://app.sendmux.ai/api/v1/mailbox/attachment-uploads/upl_123?upload_token=... type: string required: - upload_id - upload_url - method - expires_at - headers - max_size_bytes type: object ResponseMeta: additionalProperties: false properties: request_id: example: req_clxxxxxxxxxxxxxxxxxxxxxxxxx type: string required: - request_id type: object ApiErrorDetail: additionalProperties: false properties: code: description: >- Machine-readable issue code from the validator (e.g. zod issue code). example: invalid_string type: string field: description: Dot-path of the offending request field. example: recipient.email type: string message: description: Human-readable issue description. type: string required: - field - code - message type: object securitySchemes: bearerAuth: bearerFormat: API Key description: >- Sendmux API key. Use a root API key for Management API routes, or a mailbox credential for Mailbox API routes. Obtain keys from the dashboard under API Keys. scheme: bearer type: http ````