+'UploadVerifyFile': extra file verification, based on mime type, etc. Preferred
+ in most cases over UploadVerification.
+object $upload: an instance of UploadBase, with all info about the upload
+string $mime: the uploaded file's mime type, as detected by MediaWiki. Handlers
+ will typically only apply for specific mime types.
+object &$error: output: true if the file is valid. Otherwise, an indexed array
+ representing the problem with the file, where the first element
+ is the message key and the remaining elements are used as parameters to
+ the message.
+