You'll need to find the default root class for the extension and alter the Default value for the key. For example:
HKEY_CLASSES_ROOT\IrfanView.jpg
...holds the value "IrfanView JPG File". Simply edit the description in the registry and refresh DOpus. Note that if the default app changes, the description will change to whatever that app wants until you find the new key and edit it as desired.
There were some scripts that used to drift around for resetting a bunch of default handlers, but they reset the description AND the action, which I don't believe is what you want. (I'm assuming you still want to open images in IrfanView, you just don't want the stupid description.)
Bonus tip: install Command: GoRegistry (open reg-key by button or path-field) to be able to jump directly to a Registry key in the clipboard.