Hi Mike,
For almost any development task that you might want to do in your custom features you can find example code that does similar things in existing features in the mojoPortal source code. For this specific question the code for the file manager, the image gallery, or webstore all provide file upload functionality so you can study the code for one or more of those features to learn how to do it in your own feature.
If you are working with the source code you can browse the code more easily, if not you can browse our source code repository online.
Here are some links to good examples:
The EditImage page is a better example for uploading a single file whereas the FileManager is a good example of uploading multiple files at once.
Note that we use NeatUpload to support uploading multiple files at once and it also supports a progress bar during upload. However it only works in full trust hosting, in medium trust NeatUpload is disabled in web.config and degrades gracefully to the standard asp.net way of uploading which doe snot have a progress bar and does not support multiple files at once.
Hope that helps,
Joe