415 🌐 HTTP

HTTP 415 Unsupported Media Type

The server refuses the request because the body’s Content-Type isn’t one it accepts.

Seen on: JavaScript .NET

Meaning

The endpoint expects a specific body format (usually application/json) and the request declared — or defaulted to — something else, like text/plain or application/x-www-form-urlencoded.

Common causes

  • Missing Content-Type: application/json header
  • Sending form data to a JSON-only endpoint (or vice versa)
  • Wrong charset or vendor media type (application/vnd.api+json)
  • Uploading a file type the endpoint doesn’t allow

⚡ Quick fix

  1. Set the correct Content-Type header for the body you send
  2. Check API docs for the required media type
  3. With FormData in browsers, do NOT set Content-Type manually — the browser adds the boundary

Detailed fix by platform

JavaScript

  1. JSON vs multipart:
    javascript
    // JSON
fetch(url, { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(data) });
// File upload — let the browser set multipart/form-data; boundary=...
const fd = new FormData(); fd.append('file', file);
fetch(url, { method: 'POST', body: fd });

.NET

  1. ASP.NET Core [ApiController] requires JSON by default; use [FromForm] for form posts.

How to diagnose

  1. Header — What Content-Type did the client send?
  2. Endpoint — What does the endpoint consume?
  3. Body — Does the body actually match the declared type?

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