[icebraining]

In fact, HTML already has a solution for that in the form of multipart/form-data.

Instead of encoding the files directly inside JSON, you could add them as extra parts of the MIME message, and just have a reference in the JSON value (as in email, which uses a "cid:<part-identifier>" URI to inline images as such, as described in RFC2392).

You'd still need special support on the server, though.