fix: serve JavaScript assets with a stable MIME type

news/6831.bugfix.md

@@ -0,0 +1 @@
+Fix production frontend hydration on Windows when the system MIME registry maps JavaScript files to `text/plain`.

reflex/utils/precompressed_staticfiles.py

@@ -143,15 +143,18 @@ def file_response(
         response_path: str | PathLike[str] = full_path
         response_stat = stat_result
         response_headers: dict[str, str] = {}
-        media_type: str | None = None
+        media_type = (
+            "text/javascript"
+            if Path(full_path).suffix.lower() in {".js", ".mjs"}
+            else guess_type(os.fspath(full_path))[0]
+        )
 
         if self._encodings:
             response_headers["Vary"] = "Accept-Encoding"
             sidecar = self._select_sidecar(full_path, scope)
             if sidecar is not None:
                 content_encoding, response_path, response_stat = sidecar
                 response_headers["Content-Encoding"] = content_encoding
-                media_type = guess_type(os.fspath(full_path))[0] or "text/plain"
 
         response = FileResponse(
             response_path,

tests/units/utils/test_precompressed_staticfiles.py

@@ -121,6 +121,35 @@ async def test_precompressed_static_files_prefers_best_accept_encoding(
     assert await _collect_body(response, scope) == b"compressed-brotli"
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("accept_encoding", "response_name"),
+    [("identity", "app.js"), ("gzip", "app.js.gz")],
+)
+async def test_precompressed_static_files_use_javascript_mime_independent_of_system_mapping(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    accept_encoding: str,
+    response_name: str,
+):
+    """Serve JavaScript with its standard MIME type despite a host override."""
+    (tmp_path / "app.js").write_text("console.log('hello');")
+    (tmp_path / "app.js.gz").write_bytes(b"compressed-gzip")
+    monkeypatch.setattr(
+        "reflex.utils.precompressed_staticfiles.guess_type",
+        lambda _path: ("text/plain", None),
+    )
+
+    static_files = PrecompressedStaticFiles(directory=tmp_path, encodings=["gzip"])
+    response = await static_files.get_response(
+        "app.js", _scope("/app.js", accept_encoding)
+    )
+
+    assert isinstance(response, FileResponse)
+    assert str(response.path).endswith(response_name)
+    assert response.media_type == "text/javascript"
+
+
 @pytest.mark.asyncio
 async def test_precompressed_static_files_fall_back_to_identity(tmp_path: Path):
     """Keep serving the original file when no accepted sidecar is available."""