Hi Martin,
maybe its important to clarify that the respository view in Redmine isn't meant to upload files. One can link existing files in issues and wiki pages but for the most part that's it. So if you have files already in this folder you can see it in the repository view. Same is true for the Subversion repos or any other version control system. In Redmine you have the possibility to browse and link the contents of the repository and its history, look at changes and link them to issues. But there's no possibility to modify the repository in Redmine directly.
Greetings,
Bernhard