Assuming the UAS understands any extensions required by the client,
the UAS examines the body of the message, and the header fields that
describe it. If there are any bodies whose type (indicated by the
Content-Type), language (indicated by the Content-Language) or
encoding (indicated by the Content-Encoding) are not understood, and
that body part is not optional (as indicated by the Content-
Disposition header field), the UAS MUST reject the request with a 415
(Unsupported Media Type) response. The response MUST contain an
Accept header field listing the types of all bodies it understands,
in the event the request contained bodies of types not supported by
the UAS. If the request contained content encodings not understood
by the UAS, the response MUST contain an Accept-Encoding header field
listing the encodings understood by the UAS. If the request
contained content with languages not understood by the UAS, the
response MUST contain an Accept-Language header field indicating the
languages understood by the UAS. Beyond these checks, body handling
depends on the method and type. For further information on the
processing of content-specific header fields, see Section 7.4 as well
as Section 20.11 through 20.15.
