alt Hacker Newsshipping base64 in json instead of a multipart POST is very bad for stream-processing. In theory one could stream-process JSON and base64... but only the json keys prior would be available at the point where you need to make decisions about what to do with the data.
Still, at least it's an option to put base64 inline inside the JSON. With binary, this is not an option and must send it separately in all cases, even small binary...
You can still stream the base64 separately and reference it inside the JSON somehow like an attachment. The base64 string is much more versatile.