[AUCTeX] Problem accessing style files when document is split from preamble

[Nicolas Vaughan]

Hi,
I have a problem using AUCTeX style files when a documentis broken into 
a main file and a preamble.
For instance, say my preamble is contained in the following file 
preamble.tex:

% --------- preamble.tex begins here -----------
\documentclass{article}
\usepackage{csquotes}
\usepackage{tcolorbox}
% --------- preamble.tex ends here -----------


And my document document.tex is this one:

% --------- document.tex begins here -----------
\input{preamble.tex}
\begin{document}
  Lorem ipsum...
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: "preamble"
%%% End:
% --------- document.tex ends here -----------


The problem is that AUCTeX doesn't know that I am using csquotes and 
tcolorbox packages, and thus it doesn't load the corresponding style 
files. Note I have included the "TeX-master" instruction.

(Of course, if I join the two files in one, AUCTeX is aware of my 
packages and everything works just fine.)

How can I fix this?

Thanks.

Nicolas Vaughan