Share files

For every requested file:

1. Require non-empty SHAREFLARE_URL and SHAREFLARE_TOKEN. If either is unset, stop and name the missing variable.
2. Set file to the path and confirm it is a readable regular file.
3. Upload it and capture the response:

curl --fail-with-body --silent --show-error \
  --data-binary "@$file" \
  -H "Authorization: Bearer $SHAREFLARE_TOKEN" \
  -H "Content-Type: $(file --brief --mime-type -- "$file")" \
  "${SHAREFLARE_URL%/}/"

4. Return the URL printed by the service. Completion means every requested file has one returned URL.

Keep SHAREFLARE_TOKEN out of output and files.