MIME handling of multipart form data isn't working in several cases #25106
Description
WPT fetch/api/response/response-consume.html fails several cases by getting "Inappropriate MIME-type for Body" thrown by script/body.rs. The comment at https://github.com/servo/servo/blob/master/components/script/body.rs#L187 is probably relevant.
Other cases are failing on ReadableStream, but this seems entirely disjoint from that.