Getting FORMAT_CONVERSION_ERROR error trying to do Document Generation on a developer account

In case it helps, the code in question:

  const documentId = '1';
  const docBytes = readFileSync(file.path);
  // create the document object
  const document = docusign.Document.constructFromObject({
    documentBase64: Buffer.from(docBytes).toString('base64'),
    name: file.filename,
    fileExtension: 'docx',
    documentId,
  });

  const envelopeDefinition = docusign.EnvelopeDefinition.constructFromObject({
    documents: [document],
  });

  await templatesApi.updateDocument(
    authInfo.apiAccountId, //defined elsewhere
    templateId, //defined elsewhere
    documentId,
    {
      envelopeDefinition: envelopeDefinition,
    }
  );

Could you please check the value of the "file.filename"? If the extension is appended at the end of the name, please remove it. Also, please ensure that you load the docx file. If the file and its fileExtension are different, this error may occur. 
I wasn't able to reproduce the same issue using the below code with my Demo account:

  // read file
  const docBytes = fs.readFileSync(args.doc2File);

  // create the document object
  const document = new docusign.Document.constructFromObject({
    documentBase64: Buffer.from(docBytes).toString('base64'),
    name: 'OfferLetterDemo',
    fileExtension: 'docx',
    documentId: args.documentId,
  });

  const envelopeDefinition = docusign.EnvelopeDefinition.constructFromObject({
    documents: [document],
  });

  let templatesApi = new docusign.TemplatesApi(dsApiClient);
  await templatesApi.updateDocument(
    args.accountId,
    args.templateId,
    args.documentId,
    { envelopeDefinition }
  );