File uploading with multipart

Hello !

I think that for small files, it makes sense to include them inside the JSON request.

Otherwise, It would be nice to include in the standard the possibility to send a multipart requests where the first part would be application/vnd.api+json then you get the files afterwards.
The JSON part can contain metadata and information allowing the service to decide if it should read the following parts or not which can be an interesting performance boost.