Fix mimetype for PDF.js files on Windows

With windows-2022 and windows-2025 on GitHub Actions,
we get:

    qutescheme:data_for_url:128 url: qute://pdfjs/web/viewer.mjs, path: /web/viewer.mjs, host pdfjs
    webenginequtescheme:requestStarted:105 Returning text/plain data

which causes:

    JS: [qute://pdfjs/build/pdf.mjs:0] Failed to load module script: Expected a
    JavaScript module script but the server responded with a MIME type of
    "text/plain". Strict MIME type checking is enforced for module scripts per
    HTML spec.

It's unclear why we get text/plain back there in the first place (can't
reproduce on a local Windows 11 install), but it's definitely wrong, so let's
just override that problematic case.

qutebrowser/utils/utils.py

@@ -722,6 +722,11 @@ def guess_mimetype(filename: str, fallback: bool = False) -> str:
         fallback: Fall back to application/octet-stream if unknown.
     """
     mimetype, _encoding = mimetypes.guess_type(filename)
+    if os.path.splitext(filename)[1] == '.mjs' and mimetype == "text/plain":
+        # For unknown reasons, Windows (at least on GitHub Actions) wrongly
+        # gives us text/plain here.
+        return "text/javascript"
+
     if mimetype is None:
         if fallback:
             return 'application/octet-stream'

tests/unit/utils/test_utils.py

@@ -858,6 +858,7 @@ def test_chunk_invalid(n):
 @pytest.mark.parametrize('filename, expected', [
     ('test.jpg', 'image/jpeg'),
     ('test.blabla', 'application/octet-stream'),
+    ('test.mjs', 'text/javascript'),
 ])
 def test_guess_mimetype(filename, expected):
     assert utils.guess_mimetype(filename, fallback=True) == expected