> an uploader would be neat but it gets really ugly really
  > fast with many different platforms to think about.

Support webdav (or sftp). There are good free dav-clients for windows now (and all the other os' have supported webdav for years). Webdav should be easier to integrate with an existing infrastructure based around http.

edit: You might even easily create your own webdav-client for windows (the black sheep when it comes to dav-support), either working off existing dav-libraries, or possibly by throwing some money at an existing free/shareware project.