Change your input to:
<input type='file' accept='text/xml' onchange='openFile(this)'">
Apart from the syntax error in your attribute, you're trying to pass a global variable called
fileupload
to your function. But no such variable exists.
You'll also need to change your function, since the input is passed directly to the function:
openFile(fileupload) {
let input = fileupload;
...
};