Fix .mjs files served with wrong MIME type (#5724)

### Motivation

Fixes #5723, which boils down to how in Windows, `mimetypes` respect the
Windows registry.

As such under a system with `.mjs` override `Content Type` as
`text/plain`, JavaScript ES modules (`.mjs` files) like `dompurify.mjs`
were being served with MIME type `text/plain` instead of
`text/javascript`.

Compliant browsers will reject them due to strict MIME type checking for
module scripts.

### Implementation

Register the `.mjs` extension with the correct `text/javascript` MIME
type, similar to how `.js` files are already registered.

### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] If this PR addresses a security issue, it has been coordinated via
the [security
advisory](https://github.com/zauberzeug/nicegui/security/advisories/new)
process.
- [x] Pytests have been added (or are not necessary).
- [x] Documentation has been added (or is not necessary).

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: evnchn

nicegui/nicegui.py

@@ -54,6 +54,7 @@ async def __call__(self, scope, receive, send):
 
 
 mimetypes.add_type('text/javascript', '.js')
+mimetypes.add_type('text/javascript', '.mjs')
 mimetypes.add_type('text/css', '.css')
 
 static_files = CacheControlledStaticFiles(

tests/test_serving_files.py

@@ -155,6 +155,10 @@ def page():
     assert response.status_code == 200
     assert response.headers['Content-Type'].startswith('text/javascript')
 
+    response = httpx.get(f'http://localhost:{Screen.PORT}/_nicegui/{__version__}/static/dompurify.mjs', timeout=5)
+    assert response.status_code == 200
+    assert response.headers['Content-Type'].startswith('text/javascript')
+
     response = httpx.get(f'http://localhost:{Screen.PORT}/_nicegui/{__version__}/static/nicegui.css', timeout=5)
     assert response.status_code == 200
     assert response.headers['Content-Type'].startswith('text/css')