MIME Type Lookup — File Extension ⇄ MIME Type, Both Directions

Real-world use cases

• Configure nginx to serve a new file type correctly

  You added .webp images or a .wasm module and nginx is serving them as application/octet-stream, so browsers download instead of rendering. Look up the extension here (image/webp, application/wasm), then add it to your types {} block or a dedicated mime.types include, run nginx -t, and reload. WebAssembly in particular MUST be served as application/wasm or streaming compilation breaks — this table flags that exact requirement.

• Set the right Content-Type on a file download endpoint

  Your API streams a generated report and the browser opens it as gibberish text instead of downloading an xlsx. Look up xlsx, copy the full header (application/vnd.openxmlformats-officedocument. spreadsheetml.sheet), and set it plus a Content-Disposition: attachment. For text exports like CSV, copy the charset-tagged form text/csv; charset=utf-8 so non-ASCII columns survive.

• Debug why a file upload is rejected

  An upload form returns 415 Unsupported Media Type or 400. Check what MIME the client is actually sending versus what the endpoint expects. This table tells you the canonical value and the historical aliases (for example .js as text/javascript vs application/javascript), so you can match the server's allow-list exactly instead of guessing.

• Label an email or message attachment correctly

  A mail client or a chat API needs an explicit MIME type for each attachment, and the wrong one makes a PDF arrive as an unopenable blob. Look up the extension, copy the MIME, and attach with that Content-Type so the recipient's client previews it inline instead of forcing a download or hiding it entirely.

• Pick the fallback type for an unknown or generated file

  You are generating files server-side and some have no clean extension. When you genuinely cannot identify the content, the safe default is application/octet-stream (forces a download). When you can identify it, set the specific type so browsers render it. This table shows both the specific values and the octet-stream fallback in one place.

Common pitfalls

• Sending application/javascript for .js — the WHATWG standard mandates text/javascript. Browsers tolerate the old name but linters and strict servers flag it.

• Forgetting ; charset=utf-8 on text types (text/html, text/csv, application/json). JSON is UTF-8 by spec, but CSV and HTML need it or non-ASCII bytes get mojibake.

• Treating the file extension as authoritative. The extension is a hint; a renamed file still carries its original bytes. To verify the real type, read the magic bytes with the File MIME Type Inspector.

• Using application/octet-stream when a specific type exists. It forces a download and blocks inline preview — only use it deliberately, not as a lazy default.

Privacy