ASPxClientUploadControl.FileUploadStart Event

<script type="text/javascript">
     function onFileUploadStart(s, e) {
          DXUploadedFilesContainer.Clear();
     }
     function onFileUploadComplete(s, e) {
          if(e.callbackData) {
               var fileData = e.callbackData.split('|');
               var fileName = fileData[0],
                    fileUrl = fileData[1],
                    fileSize = fileData[2];
               DXUploadedFilesContainer.AddFile(fileName, fileUrl, fileSize);
          }
     }
</script>
    ...
<dx:ASPxUploadControl ID="UploadControl" runat="server" ClientInstanceName="UploadControl" Width="100%"
     NullText="Select multiple files..." UploadMode="Advanced" ShowUploadButton="True" ShowProgressPanel="True"
     OnFileUploadComplete="UploadControl_FileUploadComplete">
     <AdvancedModeSettings EnableMultiSelect="True" EnableFileList="True" EnableDragAndDrop="True" />
     <ValidationSettings MaxFileSize="4194304" AllowedFileExtensions=".jpg,.jpeg,.gif,.png">
     </ValidationSettings>
     <ClientSideEvents FilesUploadStart="onFileUploadStart" FileUploadComplete="onFileUploadComplete" />
</dx:ASPxUploadControl>

<dx:UploadedFilesContainer ID="FileContainer" runat="server" Width="100%" Height="180" 
     NameColumnWidth="240" SizeColumnWidth="70" HeaderText="Uploaded files" />

const string UploadDirectory = "~/UploadControl/UploadImages/";

protected void UploadControl_FileUploadComplete(object sender, FileUploadCompleteEventArgs e) {
     string resultExtension = Path.GetExtension(e.UploadedFile.FileName);
     string resultFileName = Path.ChangeExtension(Path.GetRandomFileName(), resultExtension);
     string resultFileUrl = UploadDirectory + resultFileName;
     string resultFilePath = MapPath(resultFileUrl);
     //Save the uploaded file
     e.UploadedFile.SaveAs(resultFilePath);
     string name = e.UploadedFile.FileName;
     string url = ResolveClientUrl(resultFileUrl);
     long sizeInKilobytes = e.UploadedFile.ContentLength / 1024;
     string sizeText = sizeInKilobytes.ToString() + " KB";
     e.CallbackData = name + "|" + url + "|" + sizeText;
}

Private Const UploadDirectory As String = "~/UploadControl/UploadImages/"

Protected Sub UploadControl_FileUploadComplete(ByVal sender As Object, ByVal e As FileUploadCompleteEventArgs)
     Dim resultExtension As String = Path.GetExtension(e.UploadedFile.FileName)
     Dim resultFileName As String = Path.ChangeExtension(Path.GetRandomFileName(), resultExtension)
     Dim resultFileUrl As String = UploadDirectory & resultFileName
     Dim resultFilePath As String = MapPath(resultFileUrl)
     'Save the uploaded file
     e.UploadedFile.SaveAs(resultFilePath)
     Dim name As String = e.UploadedFile.FileName
     Dim url As String = ResolveClientUrl(resultFileUrl)
     Dim sizeInKilobytes As Long = e.UploadedFile.ContentLength / 1024
     Dim sizeText As String = sizeInKilobytes.ToString() & " KB"
     e.CallbackData = name & "|" & url & "|" & sizeText
End Sub