On the JSON returned by your HTTP API call, you will see something like this for file upload field or image upload fields:
You will be able to download the file using a separate call like this (assuming your account name is "demo" and API call url being https://www.ragic.com/demo/sales/1?v=3&api) :
The format is:
Remember to encode your file name when you send it as an URL. Your actual file name will start after the @ character, this is to avoid file name collision.