Fix uploads with proper Content-Type header.

@siman had issues with uploads; it turns out that if you set the correct CT, or something like application/octet-stream, uploads fail. If you set an empty CT, uploads succeed.

This isn't something we're checking, it's something express-openapi does. We'll have to figure out why it does that, and make it stop.

See kogosoftwarellc/open-api#420

This is turning into a bigger thing. I'm postponing this, because we do have a workaround by setting an empty Content-Type header. @siman