This is just something that WP Multisite does by default.
You may change the file limits under "Settings"
The url to that page on your blog should look something like:
your-main-multisite-blog.com/wp-admin/network/settings.php
Look towards the bottom of the page under "upload settings"
Article ID: 37
Category: WP Empire Builder
Powered by Help Desk Software HESK, brought to you by SysAid