This is a pathological case, but causes cryptic, poorly reported failure. If $wgFileExtensions contains a value in common with $wgFileBlacklist, UploadWizard refuses to upload files of any type.
This happens because Setup.php executes
$wgFileExtensions = array_diff ( $wgFileExtensions, $wgFileBlacklist );
and this creates an array with missing integer keys. This array is encoded for JavaScript by the Xml class as an Object rather than an Array, and the call to $j.inArray() that UploadWizard uses then fails on all files submitted.
To fix this, either Setup.php should use array_values( array_diff( ... ) ), or else UploadWizard.config.php should say
'fileExtensions' => array_values($wgFileExtensions),
Version: 1.20.x
Severity: normal