Content-Type troubleshooting

Invalid MIME type

A MIME type is type/subtype, with an optional structured suffix such as application/vnd.example+json. An "invalid MIME type" error almost always means one of a small set of structural problems: a missing slash, an empty type or subtype, a broken suffix, or illegal characters.

The anatomy of a MIME type

The grammar, in one line:

type "/" subtype ["+" suffix]

Concrete examples:

• text/plain — type text, subtype plain
• application/json — type application, subtype json
• application/ld+json — type application, subtype ld, suffix json

• The slash is required. It separates the type from the subtype.
• Both sides must be non-empty tokens. A token is letters, digits, and a small set of symbols.
• A suffix is optional. It follows a plus sign and describes the structured syntax of the subtype (+json, +xml, +zip).
• Type and subtype are case-insensitive. The canonical form writes them lowercase.

Common invalid-type failures

Each example shows the broken form and the corrected form.

Missing slash

Broken: applicationjson

Without the slash there is no boundary between type and subtype, and the value cannot be a media type. An operator with a slash is usually the fix; the two words are rarely meant as one token.

Fixed: application/json

Empty type

Broken: /json

The type before the slash is empty. This happens when a value is assembled from fragments or a variable on the wrong side of the slash.

Fixed: application/json

Empty subtype

Broken: application/

A subtype must follow the slash. A trailing slash usually marks a truncated value or a template placeholder left empty.

Fixed: application/json

Broken structured suffix

Broken: application/json+

The plus sign starts a suffix, so the suffix itself must not be empty. Check that the value after + names the structured syntax (json, xml).

Fixed: application/ld+json

Illegal characters

Broken: text/*

The asterisk is a valid token in media ranges such as Accept: text/*, but the slash-star and */* forms belong to Accept headers, where clients declare what they accept. A Content-Type value must name a concrete subtype, and the asterisk is reported as an illegal character here.

Fixed: text/html

Tree prefixes and registered names

Subtype names can carry a tree prefix that signals who defined the type:

• vnd. — vendor-defined types such as application/vnd.ms-excel and application/vnd.apple.mpegurl
• prs. — personal/unregistered types
• x- — legacy unregistered convention, as in application/x-www-form-urlencoded

The validator checks the token shape of the whole subtype, including the prefix. Whether the name is actually registered is a separate question; CorrectMIME checks syntax and does not look up the IANA registry.

Canonical forms

Casing of the type, subtype, and suffix is normalized to lowercase. This is the safe canonical rewrite; the media type name itself is preserved.