It isn't well documented. In fact, it isn't documented at all, but the http://wikkawiki.org/FilesAction method allows you to put {{files}} into any Wikki page and then a registered user can upload things. It has a hidden parameter for 2MB files and limits the files which sounds smart...

$max_upload_size = "21048576";
$allowed_extensions = 'gif|jpeg|jpg|jpe|doc|xls|...';

The solution is a bit messy, but you have to manually update the files in where your _/actions/files.php_ to fix this. We set ours to 100MB upload limit and added XLS and DOC files. In the actual code in actions/files.php, I found

$max_upload_size = "2097152"; // 2 Megabyte

So it was easy to change and there was no allowed extensions at all as that got taken out. Note that when you take a Wiki update, this gets blown away, so constantly have to fix this when if you have autoupdate on from your hoster. Which I do.

The second thing to do is with "Bluehost.com":http://bluehost.com where you have to edit the php.ini file change the upload there as well, edit that in the root directory. Please note that PHP itself imposes a limit on file-uploads so you have to fix that in php.ini in your root directory. You can find two parameters and this time, it is in megabytes, so 2M:

upload_max_filesize = 100M;
post_max_size = 110M;