Hi,

The "All files" mimetype and thus the respective KDE file dialog's entry is empty. To test run Gwenview, pick Open and look at the second item of the filters combo:

https://share.kde.org/index.php/s/KkLhB9ohUGh8kzi

However, the empty entry is functional, that is, when selected all files get displayed in the dialog.

This is most likely not even KDE related but also not related to particular OS so asking here first instead of filing a bug. Maybe someone noticed something similar?
Translation related or mime database related? (my LANG=en_US.UTF-8)

Tested OS is openSUSE Leap 42.2 kernel 4.4.62-18.6-default. The same appears under Plasma, various other desktops such as XFCE assuming XDG_CURRENT_DESKTOP is KDE (unusual but possible setting on default non-Plasma setups) or when given app uses Qt's own dialog.

I am unable to say when it became like that.

Separate loosely related topic is that native file dialogs other than KDE ones have no "All files" and "All supported files" entries added for free so Qt (and KDE!) apps need to add that. I am doing this in Kexi as the issue makes the apps much less usable (example: defaulting to Krita files when in fact the default filter should be "all image files"). Native Windows dialogs have the same issue.