For certain resources, my RESTful server only accepts PUT and POST requests with JSON objects as the content body, thus requiring a Content-Type of application/json instead of application/x-www-form-urlencoded or multipart/form-data or anything else.
Malformed JSON (or lack thereof) returns a 400 with the error message taken directly from the exception raised by the JSON parser, for debugging purposes.
Which HTTP error code means that the client sent a request with an unacceptable Content-Type, even if the server could technically parse the request content?
It could be 415 Unsupported Media Type according to this list: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.16.
From the current RFC9110 HTTP Semantics
The 415 (Unsupported Media Type) status code indicates that the origin server is refusing to service the request because the content is in a format not supported by this method on the target resource.
The format problem might be due to the request's indicated Content-Type or Content-Encoding, or as a result of inspecting the data directly.
If the problem was caused by an unsupported content coding, the Accept-Encoding response header field (Section 12.5.3) ought to be used to indicate which (if any) content codings would have been accepted in the request.
On the other hand, if the cause was an unsupported media type, the Accept response header field (Section 12.5.1) can be used to indicate which media types would have been accepted in the request.