gnome-keyring how to sign a document with sea horse? (the feature is gone?)

[Weiwu <a colourful land>]

Eight years ago, one can right click a document and 'sign', which would 
produce a .sig file.

https://mail.gnome.org/archives/seahorse-list/2009-October/msg00005.html

Now I tried the same on Ubuntu 17.04 and there is no 'sign' submenu.

I can see my certificate in seahorse and I can unlock it with a PIN. Even 
when the certificate is in unlocked status, right click an document (in 
nautilous) wouldn't produce a 'sign' option.

I am guessing that the fact the certificate is in a smart-card isn't
the cause? (Inserting the smart-card causes the certificate to be
listed in seahorse.

Regards
Weiwu