fetch-data.js

fetch("https://pub.s10.exacttarget.com/xxxxxxx", {  //provide URL of the processing page
        method: "POST",
        headers: {
            "Content-Type": "application/json"
        },
        body: JSON.stringify({
            base64enc: base64enc,
            fileName: fileName,
            assetName: assetName
        })
    })
    .then(function(res) {
     window.alert("Success!");
   })
    .catch(function(err) {
     window.alert("Error!");
    });