[AUCTeX] AucTex and custom fmt files

[Julien Cubizolles]

I have some tex files using custom format files. Here is the general
structure:

--8<---------------cut here---------------start------------->8---
%&../../../vuistand-fmt
\documentclass{standalone}
\begin{document}
\begin{tikzpicture}
  some tikz code
\end{tikzpicture}
\end{document}

% Local Variables:
% coding: utf-8
% End:
--8<---------------cut here---------------end--------------->8---

I can compile it with pdflatex but C-c C-c LaTeX complains that the
tikzpicture environment is not defined: the vuistand-fmt custom format
file is not read.

I didn't find anything related to format files in the AucTeX
documentation. How do I tell it to read a given fmt file ?