* When the extension is unknown, an error is shown.
* When the extension is not written, the default extension will
be added (the default extension is different depending on
if the file format is image format or an animation format).
With this change we have moved all the file selector customization to
a new special FileExportWindow. So the file selector is used only to
select the output file (no more FileSelectorDelegate = now we can use
the native file selector for File > Export).
