The default file name extension. The returned string does not include the period. The default value is an empty string ("").
Remarks
When the user of your application specifies a file name without an extension, the file_dialog appends an extension to the file name. The extension that is used is determined by the filter and default_ext properties. If a filter is selected in the file_dialog and the filter specifies an extension, then that extension is used. If the filter selected uses a wildcard in place of the extension, then the extension specified in the default_ext property is used.