Comment 5 for bug 707368

I had a similar problem with opening cbt-files. Only cbt files in my home directory would open, and not cbt files on other drives.
Checking /etc/apparmor.d/abstractions/evince I found the following line under supported file formats:

/**.[cC][bB][rRzZ7] r,

so I changed that to

/**.[cC][bB][rRtTzZ7] r,

to account for cbt files as well as cbr, cbz, cb7.

I then restarted apparmor:
sudo /etc/init.d/apparmor restart
and now cbt files work everywhere. No idea why it worked in some cases earlier despite being missing from the list.