Support of attachment in subattribute

[To-om]

JsonInputValue is the sole InputValue that support subattribute, so attachments are mandatory in the top level fields.

Attachment can be created from FileInputValue (FIV) or from AttachmentInputValue (AIV). The idea is to permit to create AIV from Json. FIV must not be created from Json because FIV contains a reference to a file on disk and may cause security risk.
Json representation of AIV should be provided by service layer, not from the http request as it references already stored attachment in datastore and must contain coherent data (in particular hashes).