Unable to download file in mobile, if I have b64 of file

I have b64 data of a file, I want to download that file in my mobile app made using meteor. Please tell how to do this.

Till now I am able to create blank file in my mobile but when I tried to write in file then it is writing either blob of that data or b64, I think this is not correct way.

So please tell how I can download file directly in mobile from b64.

If you are unsure that this isn’t the right way then you should research a bit. Hope you’ll get the guideline. Thanks!