Thanks to the recent v4.4.1 BuddyBoard firmware the http file api works as desired: you can easily upload files to a usb stick attached to the printer. To perform bulk updates of your printer farm it’s much easier to write a simple bash script which deploys the print jobs:

#!/usr/bin/env bash

set -e

# printer settings
PRINTER_HOST="192.168.1.123"
API_KEY="ToEn8eDlR7kWIiUpVPJg"
FILENAME=myfile.gcode

# capture command stdout - http status code will be written to stdout
# progress bar on stderr
# http response (json) stored in /tmp/.upload-response
CURL_HTTP_STATUS=$(curl \
    --header "X-Api-Key: ${API_KEY}" \
    -F "file=@${FILENAME}" \
    -F "path=" \
    -X POST \
    -o /tmp/.upload-response \
    --write-out "%{http_code}" \
    http://${PRINTER_HOST}/api/files/local
)

# get result
CURL_EXITCODE=$?
CURL_RESPONSE=$(cat /tmp/.upload-response)

# success ?
if [ ${CURL_EXITCODE} -ne 0 ] || [ "${CURL_HTTP_STATUS}" -ne "201" ]; then
    echo "error: upload failed (${CURL_HTTP_STATUS})"
else
    echo "upload succeed"
fi