Testing focus() for file input fields

The following form is for testing the keyboard tabbing behavior of different form fields. Especially the <input type="file"> shows some strange behavior. Trying to set the focus on such field programmatically (see JavaScript below) doesn't seem to work across browsers. However, it does work with other input fiels, which you can try by uncommenting the second line in the JavaScript code.

window.onload = function(){
document.getElementById("file_field_1").focus();
//document.getElementById("checkbox2").focus();
}

Strangely, using the input field's id as a named anchor, does work (when triggered by keyboard):

• The link at the botom of the page will set the focus to the second file field (when pressing the enter key).
• And the submit button (via the form action) will set the focus to the first file field (when hitting the space bar).