use multipart/form-data for rocket form submit

[BouncyLlama]

changing the octet-stream here to multipart/form-data. Rocket seems to return a 404 because it doesn't expect octet-stream on a form post. Using multipart also enables a better experience w/ the swagger and rapidoc tooling where you can have a mixture of fields and files be supported there in the UI rather than an opaque blob for the request body. Should go along with https://github.com/GREsau/schemars/pull/135