IE allows users to type in the value in a file field, it's Firefox
that prevents that.
Also I think the syntax there may be wrong. I would use the name of
the file field and change it's value attribute to blank.
HTML: <input type="file" name="fileName" id="fileName" />
jQuery (using 1.3.x)
$("#button").click(function(){
$("input[name='fileName']").attr("value","");
});
You can change the attributes of any other kind of input field that
way, I can't image a file field is any different.