Here is a konqueror servicemenu item that lets you
run kompare on selected files.

Put this code into ~/.kde/share/apps/konqueror/servicemenus/kompare.desktop:

[Desktop Entry]
ServiceTypes=all/all
Actions=kompare

[Desktop Action kompare]
Name=Kompare Files
Icon=kompare
Exec=kompare -caption "%c" %i -c %U

As a refinement, it would be nice to only show this if two files are selected.

magic(5) style files are used to associate magic file signatures with mime types for KDE applications. These are located in:

KDEBASE/share/config/magic/*.magic

Look in /usr/share/templates for .desktop files that define the contents
of the:

Create New -> ...

desktop menus.