I have the infamous "Filetype not supported. Thumbnail not created." error message when I upload a JPG file and try to create a thumbnail. The image itself uploads without problems. My current sample is only 164 KB. I'm using WP 1.5 and no image plugins. My options have "jpg jpeg gif png" for the allowed file formats.
I'm running Debian and mostly using the stable distribution. I first made sure that GD was installed, but that didn't solve the problem. Then I read in the forums that the problem could be the version of PHP4, so I installed the PHP 4.3.10 from Debian unstable, as well as PHP4-gd2. The php.ini file has the "extension=gd.so" line in it, uncommented. The only places that "gd" shows up in phpinfo.php are "'--with-gd=shared,/usr' '--enable-gd-native-ttf' ".
Any ideas? Anything I can check to find out what might be stopping the thumbnail code from working?