Try changing the registry key name from "Open" to something else.
No, I didn't used "open" for my key. But nevertheless my shellhandler gets
the default action (double-click on a file), when this file type has no
assigned default action (That means all "unknown" filetypes are now opened
by my application).
Can you show us the registry branch settings that you have for your
association.
Which OS's have you tried this with? I note that the PSDK
documentation mentions that the first registry key will be the default
for versions of the shell >= 5, but that didn't seem to happen when I
tried it on XP - but I might be doing something subtly different.
There's more in-depth information in the PSDK documentation section
"Creating a File Association" on MSDN.
Dave