Downloading tflite models from server rather than using the models from assets folder in Android

I want to use the tflite models from server，but I can't download them into assets folder. So I have to load the model form sd card or other place, and how can I do that? I have tried to load the model directly, but it needs the startOffset and declaredLength of the model, which could get from AssetFileDescriptor if the model in asset folder. I don't know how to get them when the model in sd card.