Probably this is a bug , and it would be nice if you find the time to report it. I've found another-older bug about this same problem, but has not been resolved due to EOL version of Fedora.
https://bugzilla.redhat.com/show_bug.cgi?id=845855
As a workaround (this is a general script, not for decryption only - you can modify it or change it accordingly , if you want only the decryption feature)
you can use the Open with custom command script. Open a terminal and
gedit .local/share/nautilus/scripts/"Open with custom command"
Add the following content (copy-paste from here)
#!/bin/bash
var=$(zenity --entry \
--title="Add Application" \
--text="Use a custom command" \
--width="320")
if [ $? -eq 0 ] && [ "$var" ]; then
$var "$1"
else
exit 0
fi
save the file and then write in terminal
chmod 755 .local/share/nautilus/scripts/"Open with custom command"
Now, on right click, you should see an entry : scripts and there the Open with custom command. Click there and at the opened window write: /usr/bin/gpg
. It should work without problems.
Got the same issue and I thought it was the same issue that someone reported to ubuntu http://askubuntu.com/questions/519977/seahorse-decrypt-context-menu-does-not-appear-in-nautilus (but it isn't)