Content-Type troubleshooting

Content-Type header errors

A Content-Type header has one value: a media type such as application/json, optionally followed by semicolon-separated parameters such as charset=utf-8. Most "invalid Content-Type" failures are syntax errors in one of those two parts, and each part has its own fixes.

Choose your symptom

• Media type shape is wrong (missing slash, empty type or subtype, malformed suffix): Invalid MIME type
• Charset parameter is broken (missing semicolon, empty value, broken quotes, duplicates): Content-Type charset errors
• Any other parameter is broken (missing value or equals sign, bad quoting, invalid names): MIME / Content-Type parameter errors
• The multipart boundary parameter fails: multipart/form-data boundary errors

The parts of a Content-Type value

When an error message quotes a broken header, separate the header from the value first:

Content-Type: application/json; charset=utf-8

The failings live in the part after the colon. That value is two pieces of grammar: the media type (application/json) and a parameter list (charset=utf-8). An error usually belongs entirely to one piece:

• Media type errors affect the type, subtype, or structured suffix, and the whole value fails before parameters are even read.
• Parameter errors leave a valid media type but break the parameter list that follows the first semicolon.

The validator reports which piece failed and where: a missing slash is a media type error, while a parameter with no value is a parameter error. Correct the reported piece and re-check the whole value, because one preserved error can keep a strict receiver rejecting it.

Why the same header works in one place and fails in another

Receivers differ in how much they tolerate. A strict parser rejects a malformed Content-Type outright, which shows up as a refusal or a 4xx response. A lenient parser strips the broken parameter and proceeds, which is why the same header can pass one client and break another. The dangerous cases are the ambiguous ones: duplicates, where clients disagree about which value wins, and unquoted values containing spaces, where clients split parameters at different points.

Diagnosis workflow

1. Copy the header value only, without the Content-Type: field name.
2. Paste it into the Content-Type Validator and read the first reported issue.
3. Open the matching guide above and apply the fix at the source: the serializer, template, or header constant that emitted the value.
4. Re-validate the corrected value and confirm the report is clean or has only accepted warnings.
5. Re-run the failing request or client against the new header.

What CorrectMIME checks

• Media type structure: type, subtype, and structured suffix tokens
• Parameter syntax: names, values, equals signs, semicolon separators, and quoting
• Duplicate parameters and empty parameter segments
• Warnings for unusually long values

The tool validates the header value in isolation. It does not look up the IANA registry, examine message bytes, or parse a full HTTP request or email.