LXQt save dialogs don't automatically add file extensions

[Lyn Perrine]

Save dialogs don't provide an option to automatically add extensions, so even where some application understands one particular extension, saving a file in it doesn't result in an extension being added.

STEPS TO REPRODUCE
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Originally noticed this in k3b, so:
 1. Open k3b
 2. Click "Create a New Data Project"
 3. Click "Save"
 4. Note that the file name does not have the ".k3b" extension
 5. Click ok to use the default file name
 6. Open the file manager to the save location and note the file does not have the extension

EXPECTED RESULTS
================
Save dialogs should at least have a checkbox providing an option to automatically add an extension.

ACTUAL RESULTS
==============
Save dialogs have no options regarding extensions and files get saved without them unless manually added.

NOTES
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Actually, LXQt already has code to properly manage this, but if the application doesn't have matching code, then nothing is provided. KDE works around this in a non-Qt-standard way with, k3b which is why it appears to work in Kubuntu but not in Lubuntu.

There's also an additional upstream bug to allow one to toggle through the different extensions with the File Type:
https://github.com/lxqt/libfm-qt/issues/423