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>

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 ## 3.7.7.dev
+* Fix: web plugin helpers httputils.serve_resource/serve_folder ignored their mimetypes and fallback_mimetype parameters and always used the built-in mapping, so custom web plugins could not serve additional file types with a correct Content-Type
 * Fix: free-busy REPORT always failed with HTTP 400 ("FREEBUSY occurrences limit of 0 hit") when [reporting] max_freebusy_occurrence is set to 0 (limit disabled), because the limit check did not honor the disabled limit
 * Fix: time-range filter treated a VEVENT with a whole-day DURATION (e.g. P1D, P2D) as zero-length (timedelta.seconds instead of total_seconds), so such events were missing from calendar-query REPORT results
 * Fix: calendar-data expand (REPORT) left recurrence properties (e.g. RDATE) on the expanded single-occurrence VEVENTs; a single try/except around the sequential delattr() calls stopped at the first absent property (e.g. missing EXDATE), so later ones were never removed