I do not have "Open with" in the contextual menu when I select more than one photo in Windows Explorer.
edit- as a workaround I associated NEF files with C1 and did Open while having 5 selected. Only a single one appear in the import dialog.
That's probably why it doesn't work. You can't pass files to C1 via Open/Open With. Has this ever worked? Because from what I experienced, using open/openwith command with the default application opens them in different instances. If Paint is associated with NEF files, it would open 5 instances of paint. In this case C1 catches the first one that opens and then since the dialog is already open it prevents the other ones from coming through. There must be a way to code it so that C1 receives the 5 files together and not one after the other.