Hello,
When you save a json document with file then the json doc will have the file details like file name, file url etc. When you query for the docs using our different APIs such findDocumentById or any other get apis listed in our documnetation then you get the json document along with the file details if any. NOw you can download the attched file using the file URL available in the file details.
Let me know if you have any further queries.
Thanks.