For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
requirejs('[fab/fabrik]', function() {
Fabrik.addEvent('fabrik.fileupload.plupload.build.start', function(el) {
// the files are in el.options.files, which is an object of objects you need to sort by the name property
});
});