Here is how I set that value:
<telerik:radupload id="RadUpload1" runat="server" onclientadded="addTitle" onclientfileselected="uploadFileSelected" />
<script type="text/javascript" language="javascript">
function uploadFileSelected(sender, args) {
var currentIndex = sender._currentIndex - 1;
fileNameTextBox = $get('<%= RadUpload1.ClientID %>' + 'Desc' + currentIndex);
var inputValue = args.get_fileInputField().value;
var fileName = inputValue.substring(inputValue.lastIndexOf("\\") + 1);
fileNameTextBox.value = fileName;
}
function addTitle(radUpload, args) {
var curLiEl = args.get_row();
var firstInput = curLiEl.getElementsByTagName("input")[0];
//Create a simple HTML template.
var table = document.createElement("table");
table.className = 'AdditionalInputs';
//A new row for a Title field
row = table.insertRow(-1);
cell = row.insertCell(-1);
var input = CreateInput("Title", "text");
input.className = "TextField";
input.id = input.name = radUpload.getID(input.name);
var label = CreateLabel("Title", input.id);
cell.appendChild(label);
cell = row.insertCell(-1);
cell.appendChild(input);
//A new row for a Description field
row = table.insertRow(-1);
cell = row.insertCell(-1);
input = CreateInput("Desc", "text");
input.className = "TextField";
input.id = input.name = radUpload.getID(input.name);
label = CreateLabel("Description", input.id);
//label.innerHTML = firstInput.valueOf;
cell.appendChild(label);
cell = row.insertCell(-1);
cell.appendChild(input);
//Add a File label in front of the file input
var fileInputSpan = curLiEl.getElementsByTagName("span")[0];
var firstNode = curLiEl.childNodes[0];
label = CreateLabel("File", radUpload.getID());
curLiEl.insertBefore(label, firstNode);
curLiEl.insertBefore(table, label);
}
</script>