Implement any custom validation logic in the onSelect callback. This example rejects files with spaces in the name and requires a naming convention.

AjaxUploader.create(element, {
 onSelect: function (file) {
 if (/\s/.test(file.name)) {
 alert('Filenames must not contain spaces.');
 return false;
 }
 if (!/^[a-zA-Z]/.test(file.name)) {
 alert('Filename must start with a letter.');
 return false;
 }
 return true;
 }
});