Free MIME Type Lookup

How this tool computes its result

A static lookup table (`MIME_BY_EXTENSION`) hardcodes roughly 29 common extension-to-MIME-type pairs at module scope. A reverse index (`EXTENSIONS_BY_MIME`) is derived once with `reduce`, grouping every extension that shares a MIME string (e.g. both `jpg` and `jpeg` map to `image/jpeg`). The query is lowercased and a single leading "." is stripped, then checked first against the extension table, then against the MIME-type table, and reported "not found" if neither matches.

Worked example

Querying "jpg" matches the extension table and returns MIME type `image/jpeg`. Querying "image/jpeg" (the reverse direction) matches the MIME table and returns extensions `["jpg", "jpeg"]` — both keys mapping to that same MIME string. Querying "application/javascript" returns `["js", "mjs"]`.

When not to use this tool

The table covers only about 29 hardcoded extensions/MIME types — anything outside it (`.avif`, `.heic`, `.wasm`, `.parquet`, etc.) returns "No mapping found" even though it is a real, valid type; this is not backed by the full IANA media type registry.

Common mistakes

• - Typing a MIME type with parameters attached, like "application/json; charset=utf-8" — the lookup lowercases the whole string but does a strict key match against the table, so anything beyond the bare "application/json" fails to match.
• - Expecting compound extensions like ".tar.gz" to resolve — the code strips only a single leading dot and looks up the entire remaining string as one key, so ".tar.gz" never matches the "gz" entry.
• - Assuming a "not found" result means the extension or MIME type is invalid — it only means it is not one of the roughly 29 pairs hardcoded in this specific tool's table.

Related tools