MIME Type Lookup — File Extension ↔ Content-Type

This is a reference tool, not a calculator — so the “method” is the data model and the rules behind it, every record sourced. A single typed module holds 85 curated types covering 121 extensions, each record carrying its canonical type/subtype string, the extensions that map to it, a category, whether it is IANA-registered, and a sourced note where the standard and real-world server behaviour differ.

1. Normalise. The query is trimmed and lowercased. For an extension a leading *. glob or a single dot is stripped, so .PNG, png, and *.png all resolve the same.
2. Detect direction. If the query matches the RFC 6838 type/subtype grammar (it contains a / and valid characters) the tool does a reverse lookup (MIME → extension); otherwise a forward lookup (extension → MIME). Parameters after a ; are dropped, so text/html; charset=utf-8 matches text/html.
3. Forward. The extension indexes directly to its entry. Each extension is claimed by exactly one record, so the answer is unambiguous; where two types historically compete (e.g. .ico), the table records the IANA-registered string and notes the de-facto alternative — the preference order IANA-first, then Apache.
4. Reverse. The media type matches its canonical string or any recognised alias — application/javascript resolves to the .js entry whose canonical type is text/javascript, per WHATWG.
5. Fallback. A miss returns application/octet-stream — the generic binary type browsers fall back to for unknown content (WHATWG MIME Sniffing) — plus the closest fuzzy matches.

The registration label comes from the subtype prefix (RFC 6838): no prefix is the standards tree, vnd. is the vendor tree (the Office and OpenDocument types), and x-marks an unregistered de-facto type. A self-check function verifies that every record's top-level type equals its category, that every string matches the type/subtype grammar, and that no extension is claimed twice — the lookup's version of a cross-formula check.

• IANA — Media Types registry (the canonical type/subtype strings)
• WHATWG — MIME Sniffing Standard (web behaviour & octet-stream fallback)
• Apache HTTP Server — conf/mime.types (de-facto extension mapping)
• RFC 6838 — Media Type Specifications and Registration Procedures

Type strings follow the IANA registry verbatim; the IANA/de-facto labels follow RFC 6838's registration trees. The bundled table was last cross-checked against the IANA registry and Apache mime.types on 2026-06-27.