800КБ И fetch\ajax режут запрос с POST

    $.ajax({
        data: new FormData($('form')[0]),
        crossDomain: true,
        dataType: "json",
        url: 'https://xxxx.biz/api.php',
        mimeType: "multipart/form-data",
        cache: false,
        type: 'POST',
        processData: false,
        contentType: false,
        success: callback,
        error: callbackError
    })

async function postData(url, data) {
  // Default options are marked with *
  const response = await fetch(url, {
    method: 'POST', // *GET, POST, PUT, DELETE, etc.
    mode: 'cors', // no-cors, *cors, same-origin
    cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached
    credentials: 'same-origin', // include, *same-origin, omit
    headers: {
      'Content-Type': 'application/json'
      // 'Content-Type': 'application/x-www-form-urlencoded',
    },
    redirect: 'follow', // manual, *follow, error
    referrerPolicy: 'no-referrer', // no-referrer, *client
    body: data // body data type must match "Content-Type" header
  });
  return await response.json(); // parses JSON response into native JavaScript objects
}
 
postData('https://xxxx.biz/api.php', new FormData($('form')[0]));