I've googled and looked over the documenatation at http://meta.wikimedia.org/wiki/ and havn't found answers to the following:

First, some images show correctly for at least some logged-in users but show as "Missing image" when I'm not logged in. Any ideas?

Second, I'm trying to add to the list of allowable file types. http://en.wikibooks.org/wiki/Help:Images_and_other_uploaded_files is the best reference I found. It says to add, for example, $wgFileExtensions = array( 'png', 'gif', 'jpg', 'jpeg', 'ogg', 'zip', 'mpg'); to LocalSettings.php. I did that but it still won't let me upload a .mpg. It tells me The file you uploaded seems to be empty. This might be due to a typo in the file name. Please check whether you really want to upload this file.

Is there something I need to do to make the changes take effect?