UserGuide: Upload plugin
Navigation Plugin
Upload plugin provides support for uploading files :)
Security(Edit)
Upload plugin used to be part of official LionWiki distribution (before 3.0.7a) but it proved to be one big security hole. Although all known security bugs were fixed, it's principally still unsecure to allow other people to upload files to your server - especially because some server configurations can still execute uploaded code (even if it's masked).
Download(Edit)
If you're sure that your server is properly configured, you can download Upload plugin here. Simply extract the archive to plugins/ folder and that's it.