LaTeX error when formatting external data with siunitx

Question

The following example fails with ! Argument of \@iinput has an extra }. The problem does not occur when not using \input or not using \num. How can I avoid the error?

\documentclass{minimal}
\usepackage{siunitx}
\usepackage{filecontents}

\begin{filecontents*}{nline}
12345.6789
\end{filecontents*}

% Command to load the number from the simulated file
\newcommand{\rload}[1]{\input{#1}}

% Command to format the number loaded from the simulated file
\newcommand{\rnum}[1]{\num{\rload{#1}}}

\begin{document}
The formatted number is: \rnum{nline}
\end{document}

Answer 1

I'd load the file contents before passing it to \num.

\begin{filecontents*}[overwrite]{\jobname.num}
12345.6789
\end{filecontents*}

\documentclass{article}
\usepackage{siunitx}

% Command to load the number from the simulated file
\ExplSyntaxOn
\tl_new:N \l_diomidis_rload_tl

\NewDocumentCommand{\rnum}{m}
 {% #1 = file name
  \file_get:nnNTF {#1} {} \l_diomidis_rload_tl
   {% the file exists
    \num{\l_diomidis_rload_tl}
   }
   {% missing file
    \mbox{MISSING}
   }
 }
\ExplSyntaxOff


\begin{document}

The formatted number is: \rnum{\jobname.num}

The formatted number is: \rnum{xyzzzzxxx}

\end{document}

Please, be aware that minimal is not meant for examples. I changed the file name just not to risk clobbering my files; any file name (the .tex extension would be implicit if missing) is good.