MJS MIME Type

About JavaScript module files

The .mjs extension marks a file as an ES module in Node.js, but on the web it is served exactly like .js. Older server configs don’t list .mjs and send application/octet-stream, which browsers reject for module scripts. Map it to text/javascript if your server doesn’t.

Other types seen for these files: application/javascript . Accept them when reading uploads, but send text/javascript yourself.

File signature

MJS has no fixed signature: it is text without a magic number, so validate uploads by parsing the content rather than by checking bytes.

Serve .mjs with the right Content-Type

Content-Type: text/javascript; charset=utf-8

text/javascript mjs;

AddType text/javascript .mjs
AddCharset UTF-8 .mjs

@mjs path *.mjs
header @mjs Content-Type "text/javascript; charset=utf-8"

curl -sI https://example.com/file.mjs | grep -i '^content-type'
# Content-Type: text/javascript; charset=utf-8

Set the type in code

res.set('Content-Type', 'text/javascript; charset=utf-8');

import mimetypes

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

mime.AddExtensionType(".mjs", "text/javascript; charset=utf-8")
w.Header().Set("Content-Type", "text/javascript; charset=utf-8")

Accept .mjs uploads

<input type="file" accept=".mjs,text/javascript">

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
• .wasm (application/wasm): WebAssembly module
• .json (application/json): JSON

More web

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

Sources

• IANA registration: text/javascript
• MDN: Common media types

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Frequently asked questions