Fix: serve_resource/serve_folder ignore mimetypes and fallback_mimetype parameters

httputils._serve_traversable looked up the Content-Type in the
module-level MIMETYPES/FALLBACK_MIMETYPE constants instead of the
mimetypes/fallback_mimetype parameters that serve_resource() and
serve_folder() accept and pass through. The parameters exist since the
helper was extracted for use by web plugins (33fcda7c, "Extract
httputils.serve_folder"), and the sibling parameters path_prefix and
index_file are honored, but a custom web plugin passing its own
mimetype mapping (e.g. to serve .json, .ico or .mjs files with a
correct Content-Type) silently got the built-in mapping and
application/octet-stream fallback instead.

Use the parameters for the lookup. No behavior change for the built-in
web module, which relies on the defaults.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

radicale/httputils.py

@@ -209,8 +209,8 @@ def _serve_traversable(
         traversable = traversable.joinpath(index_file)
     if not traversable.is_file():
         return NOT_FOUND
-    content_type = MIMETYPES.get(
-        os.path.splitext(traversable.name)[1].lower(), FALLBACK_MIMETYPE)
+    content_type = mimetypes.get(
+        os.path.splitext(traversable.name)[1].lower(), fallback_mimetype)
     headers = {
         "Content-Type": content_type,
     }