STEPS TO REPRODUCE
1. install Thunderbird from flathub and configure your email account on it
https://flathub.org/apps/details/org.mozilla.Thunderbird
2. set Thunderbird as default email client in 'Default applications' KCM
3. open Dolphin
4. right-click on a file, hover over 'Share' and choose 'Send via email...'

OBSERVED RESULT
nothing happens

EXPECTED RESULT
A new email message is created with Thunderbird and the file used in the steps above is attached to it.

SOFTWARE/OS VERSIONS
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.26.80
KDE Frameworks Version: 5.101.0
Qt Version: 5.15.7
Graphics Platform: Wayland

ADDITIONAL INFORMATION
Output of the journal log after the provided steps:

dez 04 11:34:46 neon-laptop kwin_wayland_wrapper[33459]: error: 'attachment='file:///home/stalker/bug.txt'' is not a flatpak command. Did you mean 'document-export'?
dez 04 11:34:46 neon-laptop kwin_wayland_wrapper[33459]: See '/usr/bin/flatpak --help'
dez 04 11:34:46 neon-laptop systemd[5611]: app-flatpak-2a001b095c1f4dbb97ca91c9807e029d.scope: Couldn't move process 33459 to requested cgroup '/user.slice/user-1000.slice/user@1000.service/app.slice/app-flatpak-2a001b095c1f4dbb97ca91c9807e029d.scope': No such process
dez 04 11:34:46 neon-laptop systemd[5611]: app-flatpak-2a001b095c1f4dbb97ca91c9807e029d.scope: Failed to add PIDs to scope's control group: No such process
dez 04 11:34:46 neon-laptop systemd[5611]: app-flatpak-2a001b095c1f4dbb97ca91c9807e029d.scope: Failed with result 'resources'.
dez 04 11:34:46 neon-laptop systemd[5611]: Failed to start flatpak.