title: How to convert Source[ByteString, Any] to InputStream
body: <p>akka-http represents a file uploaded using multipart/form-data encoding as <code>Source[ByteString, Any]</code>. I need to unmarshal it using Java library that expects an <code>InputStream</code>. </p>
  
  <p>How <code>Source[ByteString, Any]</code> can be turned into an <code>InputStream</code>?</p>