saveToStorage method
存储图像, 自动返回一个Storage ID >>>
Implementation
String saveToStorage() {
String uuid = Uuid().v4();
String destFile = FilePath().imageFilename(uuid);
if (!File(destFile).existsSync()) {
File(destFile).createSync(recursive: true);
}
this.file.copySync(destFile);
return uuid;
}