Mail says Mail Attachments Iconizer is incompatible.
It may happen that Mail disables the plug-in and displays a window with a Bundle Compatibility Warning. It doesn't really mean that Mail Attachments Iconizer is incompatible with your version of Mail, but according to Apple an update to Mac OS X 10.4.1 or Mac OS X 10.5 simply “prevents previously-installed plug-ins from loading”. This may happen again with new system updates. If this is the case just check for a new release of Mail Attachments Iconizer which is compatible with your version of Mac OS X, download it and reinstall. In case the version you had installed already was compatible with your version of Mac OS X, reinstallation of the same version is of course sufficient. You do not have to worry about a reinstallation resulting in a loss of your settings or registration.
You may also receive a warning saying that Mail Attachments Iconizer could not load, that is a different problem.