Multipart request 

How do I upload a file?

`mutation CreateDocumentMutation(
  $document: DocumentInput!,
  $signers: [SignerInput!]!,
  $file: Upload!
) {
  createDocument(
    sandbox: true,
    document: $document,
    signers: $signers,
    file: $file
  ) {
  id,
    processed_at
  }
}
`