/**
 * Télécharger un fichier
 */
public function download($payload = null)
{
    global $conf, $db;

    $element = $payload['element'];     // ex: 'product'
    $parentId = $payload['parentId'];
    $ref = $payload['ref'];             // nom du fichier

    // Construire le chemin
    $dir = $conf->$element->dir_output;
    $filepath = $dir . '/' . $parentId . '/' . $ref;

    if (!file_exists($filepath)) {
        return ['File not found', 404];
    }

    // Retourner le fichier en base64
    $content = file_get_contents($filepath);
    $mime = mime_content_type($filepath);
    $base64 = base64_encode($content);

    return [[
        'filename' => $ref,
        'mime' => $mime,
        'content' => 'data:' . $mime . ';base64,' . $base64
    ], 200];
}

/**
 * Uploader un fichier
 */
public function upload($payload = null)
{
    global $conf, $user;

    $element = $payload['element'];
    $parentId = $payload['parentId'];
    $filename = $payload['filename'];
    $content = $payload['content'];  // base64

    // Décoder le contenu
    $data = base64_decode(preg_replace('#^data:.+;base64,#', '', $content));

    // Chemin de destination
    $dir = $conf->$element->dir_output . '/' . $parentId;

    // Créer le dossier si nécessaire
    if (!is_dir($dir)) {
        dol_mkdir($dir);
    }

    $filepath = $dir . '/' . $filename;

    // Sauvegarder
    if (file_put_contents($filepath, $data) === false) {
        return ['Error saving file', 500];
    }

    return [['path' => $filepath], 201];
}