I'm pretty sure the upload folder has to be within the defined 'application folder tree'... So if /rootweb/forum is the 'root' of the application, you won't be able to put the uploads above '/forum' in the tree. I could be wrong, but I'm pretty sure that will be the case. It could be in another location 'within' your application space, but I don't think it can go above it (or outside it).
To redefine the folder, the configuration is stored in "app.config" file --
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