innostudio fileuploader doesen’t shows thumbnails in bs-stepper

I built innostudio fileuploader into bs-stepper in a sub step.
Uploads and previews work correctly. The problem is showing the thumbnails when files are already called using “Preload Files”.
The preload files are correctly linked, but only a “no-image-icon” is shown.
I suspect that a lazy-loader is not running and I don’t know how to start it when calling the corresponding step.
I can execute a Javascript function by clicking on this step… but what should this function contain so that the lazyload is executed afterwards?

I’ve tried the following on click-statement on the last step-button before showing fileuploader:

function preloadImages(){
    $('input[name="filesUploadV2"]').fileuploader();
}