HTML_QuickForm - Check of uploaded file using custom function

Hello, i'm using HTML_QuickForm and HTML_QuickForm_Controller to build a mulit-page Wizard for sign-in in a community. in one of the steps users may upload one (at the moment, later more) image. So i want to check, if the uploaded file is really an image, so that my process-script will be able to save an scale it down. I've tried the following Code: $file =& $this->addElement('file', 'pic1', 'Your Pictore:'); $this->addRule('pic1', 'Not an image', 'function', 'CheckImage', 'validation'); But validation::CheckImage() is never executed. I've also tried to place the function in the HTML_QuickForm_Page and deleted the 'validation'- element of the rule, but it doesn't help. I avoid the use of checking the mime-type, b/c it made some failures with some files that are JPEG-images, but not accepted. Also i'm not really shure, what mimetypes for JPGs are... IE for example uses image/pjpeg... I couldn't find anything in the docu about such problems. Thanx for your help, Steffen