WordPress multisite has some restrictions for what types of files users can upload on the network. Fortunately, there’s an easy way to make changes to the allowed file types without having to add a plugin or change any code.
This particular network admin setting is a few clicks deep. You can find it under:
Dashboard >> Network Admin >> Settings >> Network Settings
Scroll down to Upload Settings and add or remove file types from the “Upload file types” field.
This is probably a section you’ll want to configure before opening your network up for registration. Add as many file types as you feel comfortable hosting for your users. You can also change the maximum upload file size on the network settings page.
For more information on managing your WordPress multisite network check out our WordPress Multisite Manual. The manual is a handy reference to bookmark if you’re just getting started creating and managing multisite networks.