The uploaded files are publicly accessible, but without a public file listing nobody will see your test uploads, if you don't forward someone the rather cryptic download links.
Every file will be automatically deleted after 5 minutes, but might be longer accessible due to caching.
You can find the complete source code for the client (GitHub Pages) and server-side (App Engine) implementations in the master branch of this project (which is in sync with the gh-pages branch).
The App Engine source code is located in the server/gae-python and server/gae-go folders.
Originally, the demo had been built for the Python runtime of Google App Engine, but it's since been rebuilt for the Go Runtime Environment.
You can also find an example PHP based server-side implementation under the server/php folder that can be run on any typical PHP webhosting platform.
All implementations can be run in cross-site scenarios, which means the HTML pages can be hosted on a different domain than the upload handlers.
e.g. the App Engine demo runs on jquery-file-upload.appspot.com, while the upload form is hosted on blueimp.github.com.
All implementations support every available File Upload plugin option (see Options documentation), with the exception of non-multipart and chunked uploads, which have not been implemented for the Google App Engine and Node.js versions.