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sv_allowdownload is what allows the client to request downloads from the server (again, in the form of sprays more often than not). In particular, sv_allowupload, when disabled, prevents users from uploading files to the server (in most cases, sprays). sv_allowupload and sv_allowdownload are the two convars that, when disabled, prevent sprays from being used. People are going to give you an answer like "sprays are disabled because of pornographic content", but this is the result of a bigger issue.
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