Fix mimetype handling on windows

langserve/playground.py

@@ -14,6 +14,39 @@ class PlaygroundTemplate(Template):
     delimiter = "____"
 
 
+def _get_mimetype(path: str) -> str:
+    """Get mimetype for file.
+
+    Custom implementation of mimetypes.guess_type that
+    uses the file extension to determine the mimetype for some files.
+
+    This is necessary due to: https://bugs.python.org/issue43975
+    Resolves issue: https://github.com/langchain-ai/langserve/issues/245
+
+    Args:
+        path (str): Path to file
+
+    Returns:
+        str: Mimetype of file
+    """
+    try:
+        file_extension = path.lower().split(".")[-1]
+    except IndexError:
+        return mimetypes.guess_type(path)[0]
+
+    if file_extension == "js":
+        return "application/javascript"
+    elif file_extension == "css":
+        return "text/css"
+    elif file_extension in ["htm", "html"]:
+        return "text/html"
+
+    # If the file extension is not one of the specified ones,
+    # use the default guess method
+    mime_type = mimetypes.guess_type(path)[0]
+    return mime_type
+
+
 async def serve_playground(
     runnable: Runnable,
     input_schema: Type[BaseModel],
@@ -39,7 +72,7 @@ async def serve_playground(
 
     try:
         with open(local_file_path, encoding="utf-8") as f:
-            mime_type = mimetypes.guess_type(local_file_path)[0]
+            mime_type = _get_mimetype(local_file_path)
             if mime_type in ("text/html", "text/css", "application/javascript"):
                 response = PlaygroundTemplate(f.read()).substitute(
                     LANGSERVE_BASE_URL=base_url[1:]

tests/unit_tests/test_playground.py

@@ -0,0 +1,24 @@
+import pytest
+
+from langserve.playground import _get_mimetype
+
+
+@pytest.mark.parametrize(
+    "file_extension, expected_mimetype",
+    [
+        ("js", "application/javascript"),
+        ("css", "text/css"),
+        ("htm", "text/html"),
+        ("html", "text/html"),
+        ("txt", "text/plain"),  # An example of an unknown extension using guess_type
+    ],
+)
+def test_get_mimetype(file_extension: str, expected_mimetype: str) -> None:
+    # Create a filename with the given extension
+    filename = f"test_file.{file_extension}"
+
+    # Call the _get_mimetype function with the test filename
+    mimetype = _get_mimetype(filename)
+
+    # Check if the returned mimetype matches the expected one
+    assert mimetype == expected_mimetype