curl --request POST \
  --url https://restapi.sign.plus/v2/envelope/{envelope_id}/document \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form file=@example-file

{
  "id": "<string>",
  "name": "<string>",
  "filename": "<string>",
  "page_count": 123,
  "pages": [
    {
      "width": 123,
      "height": 123
    }
  ]
}