It's the way your server detects MIME type, which seems to be "wrong", not standard. Not a bug in 4images.
You need add pjpeg MIME type in upload.php:
$this->accepted_mime_types['thumb'] = array(
"image/jpg",
"image/jpeg",
"image/pjpeg",
"image/gif",
"image/x-png",
"pjpeg",
);
never mind there is no such thing in v1.7.8