curl_mime_headers(3) on macOS
The text aff32e86d40b, as macOS 26.5, macOS 26.4, macOS 26.3, macOS 26.2, macOS 26.1, macOS 26.0, macOS 15.6, macOS 15.5, macOS 15.4, macOS 15.3, macOS 15.2, macOS 15.1, macOS 15.0, macOS 14.6, macOS 14.5 shipped it. The page shows this text now.
set a mime part's custom headers