curl --request POST \
  --url https://sandbox.api.fin.com/v1/beneficiaries/{beneficiary_id}/documents \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form doc1='@example-file'

{
  "data": {
    "files": [
      {
        "invoice1": "/wKbvfH5E_Invoice.pdf"
      },
      {
        "invoice2": "/XUOdWacK_Invoice.jpg"
      }
    ]
  }
}