WASM MIME Type

About WebAssembly module files

The fast loading path, WebAssembly.instantiateStreaming(fetch(url)), compiles while downloading, but only if the response has exactly application/wasm. With any other type it throws “Incorrect response MIME type”, and many loaders silently fall back to the slower path of downloading the whole file first.

• WebAssembly binaries often shrink by half or more with gzip or Brotli, so make sure compression is enabled for this type.

File signature

WASM files start with 00 61 73 6D ("\0asm"). The extension and the type a browser reports for an upload both come from the file name, so check these bytes on the server before trusting a file.

Serve .wasm with the right Content-Type

Content-Type: application/wasm

application/wasm wasm;

AddType application/wasm .wasm

@wasm path *.wasm
header @wasm Content-Type "application/wasm"

curl -sI https://example.com/file.wasm | grep -i '^content-type'
# Content-Type: application/wasm

Set the type in code

res.set('Content-Type', 'application/wasm');

import mimetypes

mimetypes.add_type('application/wasm', '.wasm')  # if your system doesn't know it
mimetypes.guess_type('file.wasm')[0]  # 'application/wasm'

mime.AddExtensionType(".wasm", "application/wasm")
w.Header().Set("Content-Type", "application/wasm")

Accept .wasm uploads

<input type="file" accept=".wasm,application/wasm">

The accept attribute only filters the file picker; users can still choose another file. Listing the extension as well as the type matters, because operating systems don’t always report the type you expect.

Related types

• .js (text/javascript): JavaScript
• .mjs (text/javascript): JavaScript module
• .bin (application/octet-stream): Binary data

More web

.html , .css , .svg , .webmanifest , .rss , .vtt . Look up any extension or type with the MIME type lookup.

Sources

• IANA registration: application/wasm
• MDN: Common media types

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Frequently asked questions