fetch wp (wordpress) example file upload how to avoid error 400 wp api json

wpfetch.js

// dont forget to activate your endpoint in the plugin
// $this->loader->add_action('wp_ajax_nopriv_upload_answer', $plugin_public, 'upload_answer');
// $this->loader->add_action('wp_ajax_upload_answer', $plugin_public, 'upload_answer');
//--------- js code ---------------------
const url = "/admin-ajax.php";
//el -> an file input element
const data = new FormData();
//append files
for (const file of el.files) {
  console.log('adding', file.name);
  data.append('files', file, file.name);
}

// dont forget to put the api endpoint action
data.append('action', 'upload_answer');

// any other data you want to send
data.append('case_id', 'test variable);

// jQuery.post(url, { action: 'upload_answer' });
fetch(url, { // Your POST endpoint
  method: 'POST',
  credentials: 'same-origin',

  body: data, // This is what you're sending (files and variables)
}).then(
  (response) => response.json(), // if the response is a JSON object
).then(
  (success) => console.log(success), // Handle the success response object
).catch(
  (error) => console.log(error), // Handle the error response object
);

// in the php file handle files in $_FILES and other data in $_POST