API call for thumbnail?

Yes. There is. Look at components/com_cobalt/fields/image/tmpl/output/list.php

$url = CImgHelper::getThumb(
    JPATH_ROOT . DIRECTORY_SEPARATOR . $image->get('image'), // Path to original image
    $this->params->get('params.thumbs_list_width', 100), // Max width in px
    $this->params->get('params.thumbs_list_height', 100), // Max height in px
    'image', // Subfolder for cobalt_thumbs folder
    $record->user_id, // ID of the use owns imade. optional. Use 0
    // Options how to generate thumbnail
    array(
        'mode' => $this->params->get('params.thumbs_list_mode', 1), // Mode how to generate. You can snick pick modes in parameters of the field 
        'strache' => $this->params->get('params.thumbs_list_stretch', 0), // Stratch image into new dimentions
        'background' => $this->params->get('params.thumbs_list_bg', "#000000"), // Background color
        'quality' => $this->params->get('params.thumbs_list_quality', 80))); // Quality in % 0-100

Just join it for searching file explorer windows 10 its useful to users.