Set variable in parent window from iframe

I have a parent document with an embedded iframe. Inside the iframe I have an upload field. Once the user selects a file to upload, I trigger a jQuery change event. On that event I want to set a variable in the parent window to true, so that the parent knows that the upload has started.

Does anyone know how to do this?

Was trying this, but didn't work:

var test;
$("#newsletter_email").change(function() {
  parent.window.test = true;

});

$('#send').click(function() {
    if (test) {
        alert('File has been uploaded!');
    } else {
        alert('You need to upload a file');
    }
});