It works fiine for me in FireFox, IE and Chrome
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title></title>
<script type="text/javascript">
function SetValue() {
document.getElementById("<%=TextBox1.ClientID %>").value = document.getElementById("<%=FileUpload1.ClientID %>").value;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:FileUpload ID="FileUpload1" runat="server" onchange = "SetValue()" />
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</form>
</body>
</html>