You can carry out validations OR get confirmations from user before upload. This example uses the filebatchpreupload event to trigger a confirmation dialog. It then uses the event manipulation documentation feature of the plugin to trigger an abort message based on confirmation dialog. You can read in additional data through the events as shown in the example below. For this example, the user is alerted to confirm if the number of files for upload is OK and proceed else an upload abort message is shown.

Code

<div class="file-loading">
    <input id="input-708" name="kartik-input-708[]" type="file" multiple>
</div>

<script>
$(document).ready(function() {
    $("#input-708").fileinput({
        uploadUrl: "/file-upload-batch/1",
        uploadAsync: false,
        maxFileCount: 5
    }).on('filebatchpreupload', function(event, data) {
        var n = data.files.length, files = n > 1 ? n + ' files' : 'one file';
        if (!window.confirm("Are you sure you want to upload " + files + "?")) {
            return {
                message: "Upload aborted!", // upload error message
                data:{} // any other data to send that can be referred in `filecustomerror`
            };
        }
    });
});
</script>