It looks like you are using an older version of the extension, since that isn't how it works any more. If you send me a copy of the file you updated I can take a look.
It is possible the file is too large to upload... have you tried a really small .pdf file? It also seems like that might be the wrong place to update. I'd have to look, but it should have a
$Config['AllowedExtensions']['File'] section I believe. .pdf is not an image file type, so that might be part of it as well.