I have had some issues with this widget. I wonder why there are no build-in funtions or other that allow you to validate the name, content or (mime)type of the file before actually uploading it your database. It would be neater for the user if you can show the validation error in the same way they are for normal input fields. Also it would be nice to restrict what kind of type they can upload.
The workaround now is to make a hidden button that is clicked on a onchange action of the upload widget. The file and content is then placed on the server, before it can even be processes. I think this is a not-so-nice workaround, since sometimes you want to provent users from uploading certain files in the first place. To do so, you have to upload it first... This makes no sence to me. So in my opinion it would be great if this is improved in the future versions of OS.