issues during populating FormData objects with files

Question

Ubuntu 18.04 + Firefox 70.0

I want to send files to my server PHP script by an XMLHttpRequest. I test with the simplest HTML markup and JS script, according to : https://developer.mozilla.org/en-US/docs/Web/API/FormData https://developer.mozilla.org/en-US/docs/Web/API/FormData/append and https://developer.mozilla.org/en-US/docs/Web/API/FormData/Using_FormData_Objects

I can't transfer the selected files in the declared FormData object, not with the new FormData(FormObject), nor with populating the FormData with the files in a loop.

<form id='mcp-form0' enctype="multipart/form-data" method="post">
    <input type="file" name="fichier" id="fichier" multiple="">
</form>

<script>
    let inputElement = document.getElementById('fichier');
    inputElement.addEventListener('change', handleFiles, false);

    function handleFiles() {
        let formDataConstructor = new FormData(document.getElementById('mcp-form0')),
            formDataLoop = new FormData(),
            control = "\nSelected : \n",
            fileList = this.files;
        for (let i = 0, n = fileList.length; i < n; i++) {
            let file = fileList[i];
            formDataLoop.append("userfile[]", file, file.name);
            control += "File : " + file.name + " de " + file.size + " bytes (" + file.type + ")\n";
        }

        console.log(control);
        console.log(fileList);
        console.log(formDataConstructor);
        console.log(formDataLoop);
    }
</script>

The console answer is :

Selected : 
Fichier : RetourPantalonMinouTiq.pdf de 33699 octets (application/pdf)
Fichier : rel01png.pdf de 47511 octets (application/pdf)
Fichier : rel00png.pdf de 101450 octets (application/pdf)

FileList(3) [ File, File, File ]

FormData {  }

FormData {  }

The detail of var 'control' shows that the loop made a correct job, but the 'formDataConstructor' FormData constructor-populated object, which would contain the 3 files I selected, is empty, idem the 'formDataLoop' FormData object populated in the loop.

I expected both of them to be :

FormData { File, File, File }

I can't find my mistake. Would you please help me?