415 Unsupported Media Type
Meaning
The request entity has a media type which the server or resource does not support. The server cannot process the request payload format.
When to Use
When the client sends data in a format that the server does not accept or cannot process (for instance, sending XML to an endpoint that only handles JSON).
Example
POST /api/upload with Content-Type: text/xml to a JSON API returns 415 Unsupported Media Type if the server refuses to process XML payloads.
When Not to Use
Do not use if the issue is about content that is syntactically correct but semantically unprocessable (use 422 in such cases). Also do not use if the media type is fine but something else failed (then use the appropriate code).
Source
RFC 9110