There are many different types of matching commands. But i think it's interresting to add "extension" and choose extensions in a combo or write in the textbox.
For example, i have many tif image (fichier tif in french) and i don't find this file type. in general, all non associate file are not in the combobox.
Well the filetype list is just a shortcut to save you having to know the file extension. It is built from the registry and so obviously can only contain file types that are listed in the registry. If you don't have TIF set up as a filetype then it won't be in the registry and so Opus can't add it to the list.
Just use a name match for *.tif instead.