Make a distinction between commands that can or cannot accept files with any extension

[PHPirates]

Fix #3587

Summary of additions and changes

• In Handle file extensions case sensitive #3542, the behaviour of findFile() was changed to only accept the found file if the extension is in the provided list. However, this is only correct for commands like \includegraphics, for \input any file extension is fine.

How to test this pull request

\documentclass{article}
\usepackage{graphicx}
\begin{document}
    % works
    \input{kameel.*}

    % fails
    \includegraphics{not-an-image3.nopng}

\end{document}

• Updated the documentation, or no update necessary
• Added tests, or no tests necessary