<%@ Register TagPrefix="fup" Namespace="OboutInc.FileUpload" Assembly="obout_FileUpload"%>
...
<script type="text/JavaScript">
function Rejected(fileName, size, maxSize){
var addMessage = (maxSize >= 0)?("exceeds "+maxSize+" bytes"):("-- session lost");
alert("File "+fileName+" is rejected \nIts size ("+size+" bytes) "+addMessage);
}
</script>
...
<fup:FileUploadProgress OnClientFileRejected="Rejected" runat="server">
<AllowedFileFormats>
<fup:Format Ext="*" MaxByteSize="102400"/>
</AllowedFileFormats>
</fup:FileUploadProgress>
See also working example
Allowed file extensions and sizes.