I get the the error message "The uploaded content did not match the .png file extension." as described in this bug. I fixed it as described in forum post: http://forum.dokuwiki.org/thread/1297
I'm running dokuwiki-2008-05-05 on Linux 2.6.22 i686, Apache 2.0, PHP Version 4.3.10. The same version on localhost works fine and lets me upload the same .png image the server rejects.