cmd/plume: fix prefix handling in upload subcommand
cmd/plume: fix prefix handling in upload subcommand

mjg59/mantle,mischief/mantle,dm0-/mantle,glevand/coreos--mantle,mjg59/mantle,quantum/coreos-mantle,marineam/mantle,quantum/coreos-mantle,dm0-/mantle,mischief/mantle,glevand/coreos--mantle,marineam/mantle