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>
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