415 Unsupported Media Type

415 Unsupported Media Type means the server refuses the request because the payload's format, its Content-Type, or an encoding named in Content-Encoding, is not one the target resource and method support, even though the request is otherwise well-formed. It is specifically about the format of what the client sent, which is what distinguishes it from 406, which is about the format the client asked to receive back via the Accept header.

The typical cause is a client sending XML, form-urlencoded data, or a proprietary format to an API that only accepts application/json, or omitting the Content-Type header entirely and forcing the server to guess incorrectly. File upload endpoints commonly enforce this too, rejecting an uploaded file whose declared or sniffed MIME type isn't on an explicit allowlist, a CSV importer rejecting a spreadsheet file, for instance, even though both are spreadsheet formats to a human.

A subtler cause is an incorrect or missing charset parameter on an otherwise-correct Content-Type; application/json without charset=utf-8 is technically fine by default, but a mismatched charset on a text format can cause parsing to fail. API versioning schemes that encode the version inside the media type, such as application/vnd.example.v2+json, also produce 415 when a client requests a version the server has stopped accepting, which is functionally a deprecation signal wearing a content-type error's clothing.