I've set System settings -> Default applications -> .fb2 to be opened by Okukar (by default) and Kate (as an option in Open with... menu)
Then I go to a folder with .fb2 files, click a file to open and it opens in default browser.
When I open .fb2 files via context menu -> Open with... -> Okular - all works fine. And I don't see browsers in Open with... menu, only Okular and Kate, which is correct, according to settings.

Reproducible: Always

Steps to Reproduce:
1. go to System settings -> Default applications
2. choose .fb2 extension and set it to be opened with Okular (default) or Kate (delete any other applications from list)
3. open Dolphin
4. click (or double click) a .fb2 file to open it
Actual Results:  
file is opened with default web-browser

Expected Results:  
file is opened with Okular

Please note, that though this bug is formally Minor, as it has easy workaround, it's a big useability issue.