Browsers set a real image Content-Type (image/png, image/jpeg, ...) on
the multipart avatar part, while programmatic clients often send
application/octet-stream. The part contentType tag is an allow-list for
Huma's MimeTypeValidator, which runs before the handler; broaden it so
both cases are accepted instead of being rejected with a 422.
The byte-level mimetype.DetectReader check in the handler remains the
real security gate and is unchanged.
Extend the webtest with a case that sends a part declared as image/png
and asserts it reaches the handler successfully.
cfac0773d7