kde slackware, i have no .cbr thumbs in konqueror nor dolphin, Settings/General/Preview the 'comic books' is checked, 'Skip previews for remote files above' is set at 9000 MB. 
.cbz files thumb fine, but no .cbr thumbs.
ark can see unrar fine
okular can open .cbr files, so it see's the unrar.
but the thumbnailer does not see my version of unrar in slackware its:

bash-4.3$ which unrar
/usr/bin/unrar

bash-4.3$ unrar | grep UNRAR
UNRAR 5.30 beta 6 freeware      Copyright (c) 1993-2015 Alexander Roshal

i remember reporting this before
https://bugs.kde.org/show_bug.cgi?id=232062

i've seen a log somewhere on my slackware system, something like "no suitable unrar found ..."
 

Reproducible: Always

Steps to Reproduce:
1. Open a .cbr directory and hit 'preview'
2. 
3.

Actual Results:  
no thumbnail preview for .cbr files

Expected Results:  
previewed the .cbr files

here's a picture
http://postimg.org/image/jbls23wih/