Hey, Paul...
A quick look at the help for ftype didn't reveal the detail I was looking for, but I suspect that a command without any %1 (or other percent-escapes) gets treated as if there were a %1 on the end. The issue I'd be concerned about is paths and/or file names with spaces, which I'd account for by surrounding the %1 in quotes, just like the name of the executable. If adding %1 to the ftype command doesn't help, try adding "%1", instead.
-Brandon 🙂