Fix отправка заголовка встроенным сервером в зависимости от типа asset`а
<?php
// Сервер asset`ов

if (file_exists(__DIR__ . '/../vendor/autoload.php')) {
    require_once __DIR__ . '/../vendor/autoload.php';
} else {
    require_once __DIR__ . '/../../../autoload.php';
}

$projectDir   = getenv('projectDir');
$manifestPath = getenv('manifestPath');

$assetBuilder = new Dmitrynaum\SAM\AssetServerBuilder($manifestPath, $projectDir.'/');
$assetName    = isset($_GET['asset']) ? $_GET['asset'] : null;

try{
    $content = $assetBuilder->getAssetContentByName($assetName);
} catch (Exception $e) {
    if ($e->getCode() == 404) {
        header("HTTP/1.0 404 Not Found");
    } else {
        throw $e;
    }
}

$assetType   = pathinfo($assetName, PATHINFO_EXTENSION);
$contentType = '';
switch ($fileExtension) {
    case 'js':
        $contentType = 'application/javascript';
        break;
    case 'css':
        $contentType = 'text/css;charset=UTF-8';
        break;
}

header("Content-Type:{$contentType}");
echo $content;

<?php
// Сервер asset`ов

if (file_exists(__DIR__ . '/../vendor/autoload.php')) {
    require_once __DIR__ . '/../vendor/autoload.php';
} else {
    require_once __DIR__ . '/../../../autoload.php';
}

$projectDir   = getenv('projectDir');
$manifestPath = getenv('manifestPath');

$assetBuilder = new Dmitrynaum\SAM\AssetServerBuilder($manifestPath, $projectDir.'/');
$assetName    = isset($_GET['asset']) ? $_GET['asset'] : null;

try{
    $content = $assetBuilder->getAssetContentByName($assetName);
} catch (Exception $e) {
    if ($e->getCode() == 404) {
        header("HTTP/1.0 404 Not Found");
    } else {
        throw $e;
    }
}

echo $content;
