Since the unfree unrar version has reached version 1:5.2.1-1 (currently at 1:5.2.2-1) the tumbnail preview for .cbr files failed, showing only the default document icon instead.  

Since okular had also been unable to open .cbr files since that point, i assumed a similar problem. Turns out, that since the mentioned version, unrar dropped the --version flag, leaving us only with the -v flag to identify it.

That suspicion was hardened by the fact since thumbnails were created as usual after a downgrade to unrar 1:5.1.7-1.

Based on the solution for okular from Albert Astals Cid i wrote a patch which implements a fallback check to the -v flag if the --version flag fails. Only if both checks fail, no unrar path is returned.

Reproducible: Always

Steps to Reproduce:
1. Start kde while unrar 1:5.2.2-1 or higher is installed.
2. Make sure Comicbook preview in Dolphin is enabled.
3. Navigate to a folder with at least one .cbr in it which has not yet gotten a thumbnail. (Cached thumbnails will continue to work).

Actual Results:  
Missing .cbr thumbnails will not be created.

Expected Results:  
New .cbr files getting their previews.

I will add my patch to this report. It has compiled without problems on my system with the code from the kde-runtime project as a base. I branched it from v4.14.3 of the repository.

After i restarted the patched kde, .cbr thumbnail production worked with both versions.