SUMMARY
Spectacle does not use the system setting for default file manager.

STEPS TO REPRODUCE
1. Set a different file manager as your default.
2. Save a screenshot via Spectacle.
3. Click the open containing folder button

OBSERVED RESULT
Spectacle opens Dolphin if it is installed, otherwise it may produce an error saying "Dolphin not found" or open something else such as Thunar depending on system configuration.

EXPECTED RESULT
Spectacle should open whatever the user has set as their default file manager.

SOFTWARE/OS VERSIONS
Linux Distro: Debian Trixie (13)
KDE Plasma Version: 6.3.6
KDE Frameworks Version: 6.13.0
Qt Version: 6.8.2

ADDITIONAL INFORMATION
Initially I had Dolphin installed and it was calling that every time.  When I uninstalled Dolphin, Spectacle produced an error saying Dolphin was not found.  After a reboot since then it now opens Thunar, so I guess it must load the list of available options itself on initial startup or something.  pcmanfm-qt was set as my preferred file manager in the KDE settings the entire time, so in each case it has failed to use my setting.  (I can say that xdg-open with a folder location opens my preferred file manager, so KDE has presumably set it correctly.)