If you're just getting started, you should check out the tutorials. If you're looking for information on specific widgets or features, consult the reference manual and widget gallery. Check our Redmine for forums, issues, and wiki.
If you're just getting started with Wt, these tutorials are good place to start.
This example implements a GMail-like mail composer and shows among other things how to upload files asynchronously, showing a cross-browser upload progress bar and with support for multiple files.