if(isset($_POST['article-add'])) {

$imageinfo = getimagesize($_FILES['article-image']['tmp_name']);
$arr = array('image/jpeg','image/gif','image/png');
if (array_search($imageinfo['mime'],$arr)) {
    echo "Такой формат есть";
    echo array_search($imageinfo['mime'],$arr);
}
else {
    echo "Такого формата нет";
    echo array_search($imageinfo['mime'],$arr);
}
}

<form enctype="multipart/form-data"  action="index.php" method="post">
        <label for="article-image">Картинка статьи:</label><br>
        <input type="file" id="article-image" name="article-image">
        <button name="article-add" type="submit">Отправить</button>
</form>