Arsse/tests/docroot/Feed/WithIcon/GIF.php

<?php return [
    'mime'    => "application/atom+xml",
    'content' => <<<MESSAGE_BODY
<feed xmlns="http://www.w3.org/2005/Atom">
    <icon>/Icon/GIF</icon>
    <entry>
        <title>Example title</title>
    </entry>
</feed>
MESSAGE_BODY
];