When receiving a multipart POST, parse the application/json part into an entity, and capture the file upload.

MultipartParsing.scala

pathEndOrSingleSlash {
  (post & parameter("rev")) { rev =>
    // This is very important, so we can provide a more specific
    // implicit to overcome the Json4sSupport default unmarshaller
    // that will require an `application/json` ContentType!
    import Unmarshaller._

    entity(as[Multipart.General]) { body =>

      import scala.concurrent.duration._

      onSuccess {
        body.parts.runFoldAsync((Option.empty[CreateMailer], Option.empty[TemporaryFile])) {

          case ((_, file), part) if part.entity.contentType.mediaType == `application/json` =>

            part.entity.toStrict(30 seconds).map { strict =>
              // serialization.read expects an implicit org.json4s.Formats and Manifest[T]
              (serialization.read(strict.data.decodeString(StandardCharsets.UTF_8)), file)
            }

          case ((value, _), part) =>

            val tmp = File.createTempFile("upload", "tmp")
            tmp.deleteOnExit()

            part.entity.dataBytes.runWith(FileIO.toPath(tmp.toPath)).map { _ =>
              value -> Option(TemporaryFile(part.entity.contentType.mediaType,
                                            None,
                                            tmp.length(),
                                            tmp))
            }
        }
      } {
        case (Some(create), Some(upload)) =>
          onSuccess(events ? (eventId.revise(rev), create, upload)) {
            case status: ClientError => complete(status, JNothing)
            case key: IdWithRev => complete(StatusCodes.Created -> decompose(key))
          }
        case _ => reject
      }

      reject
    } ~ entity(as[CreateMailer]) { create =>
      onSuccess(events ? (eventId.revise(rev), create)) {
        case status: ClientError => complete(status, JNothing)
        case key: IdWithRev => complete(StatusCodes.Created -> decompose(key))
      }
    }
  }
}