Version:           1.5.94 (using KDE 3.1.94 (CVS >= 20031206), Mandrake Linux Cooker i586 - Cooker)
Compiler:          gcc version 3.3.2 (Mandrake Linux 10.0 3.3.2-4mdk)
OS:          Linux (i686) release 2.4.24-0.pre2.1mdk

I think KMail should select a file icon based on what KDE think the attachment file type is instead of the declared type or mark the icon with a red question mark if there is a mismatch. KMail still tries to open the file based on the actual file type instead of the declared type.

Try attaching a png file, edit the attachment and change mime type to application/x-zip. 

When viewing the mail a zip icon will be shown and the properties show that it is a Zip archive, but opening will be done using kuickshow. There is warning about security, but nothing that indicates that the mail is seriously mailformed.