Step 2 of the upload. Call POST /v1/contacts/{id}/attachments/upload-url, upload the file with the returned presigned POST, then call this endpoint with the returned key to link the file to the contact. The file size, URL and extension are read from the stored object; a key can be linked only once. The response has the same shape as an item of GET /v1/contacts/{id}/attachments. Attachments created through the API have uploaded_by: null.