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Is it possible to limit download/upload, so only a few users can do it ?
Yes it is.
You - or whoever is the administrator of your AmpJuke installation - can edit the user(s) who should be able to download/upload music.
It's done by selecting "User adm..." (located in the lower left corner, provided you're an administrator).
When editing a user-profile, it is possible to check/uncheck if that individual user can/cannot download/upload.