Adding extension for attachments

MUST is too strong here. it makes it impossible to add new formats in the future. 

how about:
This extension defines two valid values for "content-type" are: 
 * multipart/form-data (this is the one to use for uploads)
 * application/vnd.collection+json (this is the one to use for sending regular Cj bodies)
If the content-type property is missing, clients SHOULD use "application/vnd.collection+json" when sending Cj bodies. If the content-type property is not supported and/or the provided value is not understood by the client, the client MUST use "application/vnd.collection+json" when sending Cj bodies.

adjust as you see fit.