When trying to save a file without extension, the file dialog is supposed to automatically append the extension of the selected filter. However, this is currently broken on linux and OSX: https://bugreports.qt.io/browse/QTBUG-27186
A workaround is most likely needed, seeing how the bug is known since 2006. I'm not sure whether it has been fixed in Qt5 but I wouldn't be surprised if it isn't.
A possible workaround is presented in a related bug: https://bugreports.qt.io/browse/QTBUG-11352
Until the status on Qt5 is known, I'll postpone working on this.
Yep, I can confirm this is not yet fixed in Qt 5.5.1 yet apparently.
lurchi on IRC says that it may be fixed in Qt 5.7.1 - is this something you could check sometime, Uwe?
This seems to be related to Qt's choices when KDE is used.
After installing kwin, plasma, breeze on my box I can force the behaviour lurchi was seeing like this:
So it's indeed some desktop/theme specific thing apparently. We should find out if we can force Qt5 (via C++ code in PV) to always show/use that checkbox regardless of desktop.