Warn and fix missing mime types (#1050)

Author: Archmonger

Diff for: docs/source/about/changelog.rst

@@ -23,6 +23,10 @@ more info, see the :ref:`Contributor Guide <Creating a Changelog Entry>`.
 Unreleased
 ----------
 
+**Changed**
+
+- :pull:`1050` - Warn and attempt to fix missing mime types, which can result in ``reactpy.run`` not working as expected.
+
 **Fixed**
 
 - :issue:`930` - better traceback for JSON serialization errors (via :pull:`1008`)

Diff for: src/py/reactpy/reactpy/backend/__init__.py

@@ -0,0 +1,22 @@
+import mimetypes
+from logging import getLogger
+
+_logger = getLogger(__name__)
+
+# Fix for missing mime types due to OS corruption/misconfiguration
+# Example: https://github.com/encode/starlette/issues/829
+if not mimetypes.inited:
+    mimetypes.init()
+for extension, mime_type in {
+    ".js": "application/javascript",
+    ".css": "text/css",
+    ".json": "application/json",
+}.items():
+    if not mimetypes.types_map.get(extension):  # pragma: no cover
+        _logger.warning(
+            "Mime type '%s = %s' is missing. Please research how to "
+            "fix missing mime types on your operating system.",
+            extension,
+            mime_type,
+        )
+        mimetypes.add_type(mime_type, extension)