Fix mimetype handling on windows (#259)

This PR fixes handling of mimetypes on windows machines:

Needed due to issue: https://bugs.python.org/issue43975

Resolves issue: #245

Chose not to use mimetypes.init since that's using a singleton pattern
and worried that users will be likely to run into issues since the
server may be parsing files which may also end up using mimetype
guessing based on file extension

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