System installed icons (those installed by root/administrator) are usually found in /usr/share/icons.
User installed icons (those you installed yourself) are usually found in your user's kde folder. I'm not sure in other distributions, but in Kubuntu, that would be in /home/username/.kde/share/icons.
Also, different icon themes use different icons for mimetypes, and are arranged according to icon size, so it would be something like /usr/share/icons/icon_theme/icon_size/mimetypes or /home/username/.kde/share/icons/icon_theme/icon_size/mimetypes. Where icon_size would be something like 16x16, 22x22, 32x32, etc.