Reference
MIME Types Reference
MIME types and Content-Type notes for API payload debugging.
Overview
MIME types describe the format of request and response bodies. In API debugging, the important question is whether the server and client agree on what the body contains. A wrong Content-Type can turn valid data into a parsing failure.
Debugging Reference Table
| MIME type | Typical body | API debugging note |
|---|---|---|
| application/json | JSON object, array or primitive | Required for most JSON request bodies and useful before calling response.json(). |
| text/plain | Plain text | Do not assume it can be parsed as JSON even when it looks structured. |
| text/html | HTML document | Often means a redirect, error page or login page was returned instead of API JSON. |
| application/x-www-form-urlencoded | Form encoded key-value pairs | Spaces and plus signs may need different handling than JSON. |
| multipart/form-data | File upload or mixed form fields | Boundary metadata is required; do not manually hardcode it in most browser requests. |
| application/octet-stream | Binary data | Treat as bytes, not text. |
| image/svg+xml | SVG XML | Useful for assets but should not be parsed like JSON. |
| application/problem+json | Standardized error object | Parse as JSON, but expect an error shape such as title, detail and status. |
Reference Table Coverage
- Request Content-Type headers.
- Response body parsing decisions.
- HTML returned to JSON clients.
- Form encoding versus JSON encoding.
API Debugging Examples
- A POST request sends a JSON string but omits Content-Type: application/json.
- A browser fetch receives text/html because an auth redirect returned a login page.
- A problem+json response should be parsed as structured error data.
Common Mistakes
- Calling response.json() without checking Content-Type.
- Manually setting multipart boundaries in browser upload requests.
- Assuming text/plain means the server did not return useful error details.
- Sending URL-encoded form data to an endpoint expecting JSON.
FAQ
Should I always trust the Content-Type header?
It is the best first signal, but servers can mislabel responses. If parsing fails, inspect both headers and body.
Why do I get HTML from an API endpoint?
Common causes include redirects, authentication pages, proxy errors and framework error pages.
Is application/problem+json valid JSON?
Yes. It is a JSON-based format commonly used for structured API error responses.