Specify the media type expected by the server

How would you specify the media type accepted by the server for a particular form?
Two possibilities I can think of:

• mimic HTML and use an enctype field
• mirror content negotiation and use an accept field. In this case that would mean what the server accepts as the body of the request.

While the latter seems more elegant, it might be confusing. Using the familiar enctype name might be more pragmatic.