Set default extension for save dialog file filter

The code update sets the default extension to the first pattern in the save dialog file filter when options.Filters is populated. This improvement applies to Windows platform interface function for 'Save File Dialog' and provides a more intuitive user experience.

v3/pkg/application/dialogs_windows.go

@@ -168,6 +168,11 @@ func (m *windowSaveFileDialog) show() (chan string, error) {
 		Folder:      defaultFolder,
 	}
 
+	// Original PR for v2 by @almas1992: https://github.com/wailsapp/wails/pull/3205
+	if len(options.Filters) > 0 {
+		config.DefaultExtension = strings.TrimPrefix(strings.Split(options.Filters[0].Pattern, ";")[0], "*")
+	}
+
 	result, err := showCfdDialog(
 		func() (cfd.Dialog, error) {
 			return cfd.NewSaveFileDialog(config)