What is the MIME type of .wasm files?
The MIME type (media type) for WebAssembly files with the .wasm extension is application/wasm. WebAssembly.instantiateStreaming() only works when the server sends exactly application/wasm; otherwise you get “Incorrect response MIME type”.
Content-Type header
Content-Type: application/wasmServer configuration
| Server | Configuration |
|---|---|
Nginx (mime.types or types {}) | application/wasm wasm; |
Apache (.htaccess) | AddType application/wasm .wasm |
IIS (web.config) | <mimeMap fileExtension=".wasm" mimeType="application/wasm" /> |
| Node.js / Express | res.type('wasm') or res.setHeader('Content-Type', 'application/wasm') |
| Amazon S3 | Set Content-Type: application/wasm on the object (metadata), or pass --content-type application/wasm to aws s3 cp |
Should .wasm files be compressed?
Yes. WebAssembly is uncompressed data, so enabling gzip or Brotli for application/wasm typically saves 50–80 % of the transfer size. Make sure your server’s compression list includes this type — check with the website compression test.
Why the MIME type matters
Browsers decide what to do with a response — render, run, play or download — from the Content-Type header, not the file extension. With X-Content-Type-Options: nosniff, a wrong type causes stylesheets and scripts to be blocked outright. For uploads, never trust the type sent by the client; check the file’s content (its magic bytes) on the server.
Frequently asked questions
What is the MIME type for .wasm?
application/wasm.How do I set the Content-Type for .wasm in Nginx?
application/wasm wasm; is listed in the types block (usually in /etc/nginx/mime.types) and reload Nginx.