Try the value
property, like this:
var fu1 = document.getElementById("FileUpload1");
alert("You selected " + fu1.value);
NOTE: It looks like FileUpload1
is an ASP.Net server-side FileUpload control.
If so, you should get its ID using the ClientID
property, like this:
var fu1 = document.getElementById("<%= FileUpload1.ClientID %>");