'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
-object $mime: the uploaded file's mime type, as detected by MediaWiki. Handlers
+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, and indexed array
+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.