How can I add a file to a form?

Question

JavaScript

Hello!
I have a dead end, I need to add a file to the form, I do this:
There is a form

<form name="test">
        <input type="file" id="files" accept=".docx" value=""/>
        <button id="loadButton">load</button>
    </form>

let fileInputElement = document.getElementById('files');
        let container = new DataTransfer();
        let data = new Blob();
        let file = new File([data], "y9ai4V96h1oMTyfUZrBsAFb8sopJ79PSxxqg7FdH.docx",{type:"application/vnd.openxmlformats-officedocument.wordprocessingml.document", lastModified:new Date().getTime()});
        container.items.add(file);
        fileInputElement.files = container.files;
        var size = file.size;
        console.log(size);

What am I doing wrong?
And so I tried

var formData = new FormData();
var file_obj = document.querySelector('input[type="file"]').files[0];
formData.append("file", file_obj, "y9ai4V96h1oMTyfUZrBsAFb8sopJ79PSxxqg7FdH.docx");

Doesn't work and that's it.

Answer 1

Firstly, it is not clear where you take the file for insertion from. What is in the data variable and where does it get there from?
Secondly, you completely misunderstand the purpose of the FormData and DataTransfer constructors.

Answer 2

For security reasons, the file field cannot be manually assigned a value. And all)