In Save As dialog, set default file extension from the signal received.
This is an alternative proposal to fix the BUG:440122, different from my previous !1379 (closed).
As proposed in the discussion there, instead of adding the file extension to the returned file name (in case it is missing), now the file open dialog has the slot to catch the file filter selection where it infers the extension and then can set it as default. As a result, file open dialog should always return a file name with extension, transparently checked for overwrite. This is important in the sandboxed environments where exactly the returned file path can be written and no other. The sandboxed environments will crash if we add the file extension later and attempt to write there.