Lion Server wireless filesharing for iPad, what are the file type limitations?
There doesn't appear to be any file-type limitation. WebDAV file sharing is enabled in the new Server.app under the File Sharing Service. Click the share you want, and then check "Share with iOS Devices (WebDAV)." 

There isn't anything specific about file types, and Server.app seems to treat WebDAV as just another file-sharing protocol.