Custom Validation

<div id="myUploader" class="mu-uploader">...</div>
<div id="validationMsg"></div>

var uploader = new MultipleUpload(document.getElementById('myUploader'), {
 uploadUrl: '/api/upload',
 onSelect: function(file) {
 // Reject files with spaces in the name
 if (/\s/.test(file.name)) {
 showError(file.name + ': file names must not contain spaces.');
 return false;
 }
 // Reject files over 3 MB
 if (file.size > 3 * 1024 * 1024) {
 showError(file.name + ': file exceeds 3 MB limit.');
 return false;
 }
 return true;
 }
});

Rules the built-in options cannot express

onSelect gives you the files before they enter the queue, so any rule you can write in JavaScript can gate them: a filename convention, a required document number, a combination of types, a check against something already on the page.

Explain the rejection

A custom rule is invisible to the user until they break it, and unlike a size limit they cannot guess what it was. Whatever your check rejects, say why in the same breath — otherwise a file that simply fails to appear reads as a broken control rather than a rule.