Are you getting nacks starting from the very first packet (initialization) or for consequent data? i.e. the former would look strange as it means any upload should be failing, while the later is logicac if we're not using proper upload->type - eg. pebble just cannot parse incoming bytes in accordance with specified type and hence is failing and nacking the transfer.
I presume you've been trying with UploadTypeSystemResources? Did you try feeding the file to pbpack tool to check whether it's in proper binary resource format pebble understands?