I am using the AjaxUpload package to allow users to upload video files to a folder. Although I have included the AjaxUploadRequired hook, the form can still be submitted without uploading a video, and if the video is in the process of uploading when the submit button is pressed, an alert pops up that just asks the user if they are sure they would like to leave the page because it might cause their form to reset.
Did you open the snippet “AjaxUploadRequired” in the MODX manager and replace the snippet content with the content from the file core/components/ajaxupload/elements/snippets/ajaxuploadrequired.hook.php?
When I test it, this fixed the issue. (There seems to be a typo in the installation script, so that the snippet “AjaxUploadRequired” gets the content from the wrong file.)
Do you have the current version (1.6.3-pl) of AjaxUpload installed?
You can see the version of the installed extras in the menu “Extras” → “Installer”.