POST'ing a Blob with an XHR to a custom scheme handler is broken Before the upcoming fix for https://bugs.webkit.org/show_bug.cgi?id=191362, POSTing with XHRs was broken in general. Getting the blobs to work is actually a huge task, so lets track it separately.
When this is fixed, there's an API test all ready to go inside WKURLSchemeHandler-1.mm that is commented out 👍