Steps to reproduce the problem:
Run following code :

<script>
let file = new File(['testestestest'], "test.ext");
console.log('Client-side size : ', file.size);
fd.append("file", file);

let request = new XMLHttpRequest();
request.open("POST", "server_script.php");
request.send(fd);
request.onreadystatechange = function(e) {
    if (this.readyState === XMLHttpRequest.DONE) {
        if (this.status === 200) {
            let obj = JSON.parse(this.responseText);
            console.log('Server-side size :', obj.files.file.size);
        }
    }
}
</script>

Server script (server_script.php) :

<?php
header('Content-Type: application/json');
echo json_encode(['files' => $_FILES]);

What is the expected behavior?
Logging "Client-side size : 13", then "Server-side size : 13"

What went wrong?
Logging "Client-side size : 13", then "Server-side size : 0"
Sent file is empty.

Did this work before? N/A 

Chrome version: 62.0.3202.70  Channel: stable
OS Version: 11.1.1
Flash Version: