@andymeakin. Thanks for posting. I just replied to your email about 10 minutes ago. I didn’t realize that you posted for assistance on the forum as well. In the email, you didn’t mention this diagnostic information, but having it here is very useful.
The diagnostic information reveals why you are having this error. Although you have PHP 5.3.8, your PHP setup does not have any means to detect the mime type of a file. If it was working before, that means that the fileinfo_mime_type was enabled before. As of PHP 5.3, fileinfo_mime_type is enabled by default. So, I suspect that someone has disabled it without your knowledge. Once you enable it in your PHP config settings, everything will work as expected.