create static method
Create a file storage instance with specified category as root directory.
If the directory does not exist, it will be created.
Implementation
static Future<FileStorageImpl> create(String category) async {
final Directory rootDir = await getApplicationDocumentsDirectory();
final Directory categoryDir = Directory(join(rootDir.path, category));
if (!(await categoryDir.exists())) {
await categoryDir.create();
}
return FileStorageImpl._(categoryDir);
}