Actually the question of how the grocery CRUD callback works is at the documentation: Tutorial - Using Callbacks .
Now about your request for the multiple files upload is not possible to be done with a simple callback. This is a more complicated task and you need to change the groceryCRUD library in order to achieve this. The best way is to actually create a new "type" (for example "upload_file_multiple") and create a new functionality that grocery CRUD doesn't have. If you take the time and create this functionality it would be good if you share the code to the community as well.
At the meanwhile (as @BernalCarlos already answered) , there is a post at the forums that our friend Victor Halauko created . It is an extension in order to have the multiple uploads in grocery CRUD. You can find this topic here