Multiple files can be read via the Filepicker widget at a time. If say you have 5 files read by the Filepicker widget then you can create a submit button and configure the onClick event to iterate from 0 to 4 i.e. indexes for the files in the Filepicker widget and invoke your upload query like
{{ upload_query.run(undefined, undefined, { fileIndex: i }) }}
In the upload query, you can reference the ith file like:
{{ FilePicker1.files[this.params.fileIndex].data }}