\newcommand{\attachment}[1]{%
  %  Check to see if #1 is empty (https://tex.stackexchange.com/a/53091/5764)
  \if\relax\detokenize{#1}\relax
    % <empty>
  \else% <not empty>
    \stepcounter{attachment}%
    \listadd{\attachments}{#1}%
  \fi
}