Dart APIunself_fileXPlatformFileExtension XPlatformFileExtension extension Null safety on PlatformFile Properties xFile → XFile read-only unself_file unself_file XPlatformFileExtension extension unself_file library Classes CrossFileTestOverrides FilePicker FilePickerResult PlatformFile XFile XFilePicker XPlatformFile XZipDecoder Extensions XPlatformFileExtension Constants defaultDialogTitle Enums FilePickerStatus FileType Properties xFile