How to keep filecontents* from getting confused when it encloses \begin{document}..\end{document}?

Question

I TeX the following code with pdflatex --shell-escape but the compilation got paused because of an error as the produced prepare.tex does not contain \end{document}.

\documentclass{article}
\begin{filecontents*}{prepare.tex}
\documentclass{beamer}
\usepackage{graphicx}
\usepackage[active,tightpage,graphics]{preview}
\PreviewBorder=0pt

\begin{document}
\begin{frame}
    \only<1>{\includegraphics{example-image-a}}
    \only<2>{\includegraphics{example-image-b}}
\end{frame}
\end{document}
\end{filecontents*}


\usepackage{animate}
\begin{document}
    \immediate\write18{pdflatex.exe prepare.tex}
    \animategraphics{10}{prepare.pdf}{}{}
\end{document}

How to keep filecontents* from getting confused when it encloses \begin{document}..\end{document}?

Bonus question (rather than I create a new question for it): How can I force filecontents to override the specified file if it already exists?

Edit: Actually there is no problem with filecontents. The source of problem is the extension .pdf in the \animategraphics. Now unfortunately, the title is misleading.

Answer 1

The following example works.

My changes

1. The argument of \animategraphics must NOT have a file extension.
2. I load the package filecontents to answer your bonus question.

Here the mwe:

\documentclass{article}
\usepackage{filecontents}
\usepackage{graphicx}
\begin{filecontents*}{prepare.tex}
\documentclass{beamer}
\usepackage{graphicx}
\usepackage[active,tightpage,graphics]{preview}
\PreviewBorder=0pt

\begin{document}
\begin{frame}
    \only<1>{\includegraphics{example-image-a}}
    \only<2>{\includegraphics{example-image-b}}
\end{frame}
\end{document}
\end{filecontents*}


\usepackage{animate}
\begin{document}
    \immediate\write18{pdflatex.exe prepare.tex}
    \animategraphics{10}{prepare}{}{}% <======= MUST NOT HAVE EXTENSION!
\end{document}

_{tested with Ubuntu+TeXLive}