class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
Your system doesn't find the correct MIME type. Maybe Fileinfo PECL is wrongly set up.
Try to deactivate some methods in the class code, to see which one work:
// these are the different MIME detection methods. if one of these method doesn't work on your
// system, you can deactivate it here; just set it to false
$this->mime_fileinfo = true; // MIME detection with Fileinfo PECL extension
$this->mime_file = true; // MIME detection with UNIX file() command
$this->mime_magic = true; // MIME detection with mime_magic (mime_content_type())
$this->mime_getimagesize = true; // MIME detection with getimagesize()
Try to deactivate some methods in the class code, to see which one work:
Start by setting mime_fileinfo to false.