curl
  -F "payload=</tmp/upload_file_payload.json"
  -F "file=@/tmp/file.pdf"
   -v https://host/api

val httpEntity = HttpEntity(MediaTypes.`application/octet-stream`, file, 100000)
//    val httpEntity = HttpEntity.fromPath(ContentType.apply(MediaTypes.`application/octet-stream`), Paths.get(file.getAbsolutePath))
val fileFormData = Multipart.FormData.BodyPart.Strict("file", httpEntity, Map.empty)
val jsonFormData = Multipart.FormData.BodyPart.Strict("payload", payload, Map.empty)


//      Multipart.FormData.Strict(scala.collection.immutable.Seq(jsonFormData, fileFormData)).toEntity()
val entity = Multipart.FormData( Source(List(jsonFormData, fileFormData))).toEntity()
val httpRequest = HttpRequest(HttpMethods.POST, uri = uri, entity = entity)

val httpEntity = HttpEntity(MediaTypes.`application/octet-stream`, file, 100000).toStrict(10.seconds)(mat)   // I had to convert this into strict: which adds Content-Length and tells it is not streaming
val fileFormData = Multipart.FormData.BodyPart.Strict("file", Await.result(httpEntity, 10.seconds), Map.empty)  // used Await here to get httpEntity from Future, this might be made better
val jsonFormData = Multipart.FormData.BodyPart.Strict("payload", payload, Map.empty)

val entity = Multipart.FormData(jsonFormData, fileFormData).toEntity() // Corrected this signature
val httpRequest = HttpRequest(HttpMethods.POST, uri = uri, entity = entity)